diff --git a/docs/conf.py b/docs/conf.py
index 737360c..2eb3468 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -70,7 +70,7 @@
 # The short X.Y version.
 version = "0.6"
 # The full version, including alpha/beta/rc tags.
-release = "0.6.0-dev.27+pr.93"
+release = "0.6.0-dev.28+pr.94"
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/docs/releasehistory.rst b/docs/releasehistory.rst
index 9a159e2..cd2f14d 100644
--- a/docs/releasehistory.rst
+++ b/docs/releasehistory.rst
@@ -104,6 +104,7 @@ Version 0.6.0-dev
 - Improved time index check in time-dependent models (0.6.0-dev.24+pr.90, 0.6.0-dev.25+pr.91)
 - Changed the output type of `adtk.data.split_train_test` from a 2-tuple of lists to a list of 2-tuples (0.6.0-dev.26+pr.92)
 - Removed `adtk.data.resample` because its functionality is highly overlapped with pandas resampler module (0.6.0-dev.27+pr.93)
+- Made `adtk.data.expand_event` accept events as pandas Series/DataFrame (0.6.0-dev.28-pr.94)
 
 Version 0.5.5 (Feb 24, 2020)
 ===================================
diff --git a/setup.cfg b/setup.cfg
index fbddf33..fef8a2d 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,6 @@
 [metadata]
 name = adtk
-version = 0.6.0-dev.27+pr.93
+version = 0.6.0-dev.28+pr.94
 author = Arundo Analytics, Inc.
 maintainer = Tailai Wen
 maintainer_email = tailai.wen@arundo.com
diff --git a/src/adtk/__init__.py b/src/adtk/__init__.py
index 32448ab..0f8ea88 100644
--- a/src/adtk/__init__.py
+++ b/src/adtk/__init__.py
@@ -20,4 +20,4 @@
 
 """
 
-__version__ = "0.6.0-dev.27+pr.93"
+__version__ = "0.6.0-dev.28+pr.94"
diff --git a/src/adtk/data/_data.py b/src/adtk/data/_data.py
index 1c11455..f447ee6 100644
--- a/src/adtk/data/_data.py
+++ b/src/adtk/data/_data.py
@@ -1,8 +1,6 @@
 """Module is for data (time series and anomaly list) processing.
 """
 
-from functools import reduce
-from math import gcd
 from typing import Dict, List, Optional, Tuple, Union, overload
 
 import numpy as np
@@ -228,8 +226,8 @@ def to_events(
         For example, DatetimeIndex(['2017-01-01', '2017-01-02', '2017-01-03',
         '2017-01-04', '2017-01-05'], dtype='datetime64[ns]', freq='D') has
         daily frequency. If freq_as_period=True, each time point in the index
-        represents that day. Otherwsie, each time point represents the
-        instantaneous time instance of 00:00:00 on that day.
+        represents that day (24 hours). Otherwsie, each time point represents
+        the instantaneous time instance of 00:00:00 on that day.
 
         Default: True.
 
@@ -313,6 +311,8 @@ def to_events(
                     for start, end in zip(starts[:, 0], ends[:, 0])
                 ]
     else:
+        if labels.columns.duplicated().any():
+            raise ValueError("Input DataFrame must have unique column names.")
         return {
             col: to_events(labels[col], freq_as_period, merge_consecutive)
             for col in labels.columns
@@ -373,10 +373,10 @@ def to_labels(
         '2017-01-04', '2017-01-05'], dtype='datetime64[ns]', freq='D') has
         daily frequency. If freq_as_period=True, each time piont represents
         that day, and that day will be marked positive if an event in the event
-        list overlaps with the period of that day. Otherwsie, each time point
-        represents the instantaneous time instance of 00:00:00 on that day, and
-        that time point will be marked positive if an event in the event list
-        covers it.
+        list overlaps with the period of that day (24 hours). Otherwsie, each
+        time point represents the instantaneous time instance of 00:00:00 on
+        that day, and that time point will be marked positive if an event in
+        the event list covers it.
 
         Default: True.
 
@@ -446,6 +446,7 @@ def expand_events(
     lists: List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]],
     left_expand: Union[pd.Timedelta, str, int] = 0,
     right_expand: Union[pd.Timedelta, str, int] = 0,
+    freq_as_period: bool = True,
 ) -> List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]:
     ...
 
@@ -457,35 +458,64 @@ def expand_events(
     ],
     left_expand: Union[pd.Timedelta, str, int] = 0,
     right_expand: Union[pd.Timedelta, str, int] = 0,
+    freq_as_period: bool = True,
 ) -> Dict[str, List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]]:
     ...
 
 
+@overload
 def expand_events(
-    lists: Union[
+    lists: pd.Series,
+    left_expand: Union[pd.Timedelta, str, int] = 0,
+    right_expand: Union[pd.Timedelta, str, int] = 0,
+    freq_as_period: bool = True,
+) -> pd.Series:
+    ...
+
+
+@overload
+def expand_events(  # type:ignore
+    lists: pd.DataFrame,
+    left_expand: Union[pd.Timedelta, str, int] = 0,
+    right_expand: Union[pd.Timedelta, str, int] = 0,
+    freq_as_period: bool = True,
+) -> pd.DataFrame:
+    ...
+
+
+def expand_events(  # type:ignore
+    events: Union[
         List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]],
         Dict[
             str, List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]
         ],
+        pd.Series,
+        pd.DataFrame,
     ],
     left_expand: Union[pd.Timedelta, str, int] = 0,
     right_expand: Union[pd.Timedelta, str, int] = 0,
+    freq_as_period: bool = True,
 ) -> Union[
     List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]],
     Dict[str, List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]],
+    pd.Series,
+    pd.DataFrame,
 ]:
-    """Expand time windows in an event list.
+    """Expand duration of events.
 
     Parameters
     ----------
-    lists: list or dict
-        A list of events, or a dict of lists of events.
+    events: list, dict, pandas Series, or pandas DataFrame
+        Events to be expanded.
 
         - If list, a list of events where an event is a pandas Timestamp if it
           is instantaneous or a 2-tuple of pandas Timestamps if it is a closed
           time interval.
         - If dict, each key-value pair represents an independent list of
           events.
+        - If pandas Series, it is binary where 1 represents events cover this
+          time point.
+        - If pandas DataFrame, each column is treated as an independent Series.
 
     left_expand: pandas Timedelta, str, or int, optional
         Time range to expand backward.
@@ -505,9 +535,22 @@ def expand_events(
 
         Default: 0.
 
+    freq_as_period: bool, optional
+        Whether to regard time index with regular frequency (i.e. attribute
+        `freq` of time index is not None) as time intervals. Only used when
+        input events is pandas Series or DataFrame.
+
+        For example, DatetimeIndex(['2017-01-01', '2017-01-02', '2017-01-03',
+        '2017-01-04', '2017-01-05'], dtype='datetime64[ns]', freq='D') has
+        daily frequency. If freq_as_period=True, each time point in the index
+        represents that day (24 hours). Otherwsie, each time point represents
+        the instantaneous time instance of 00:00:00 on that day.
+
+        Default: True.
+
     Returns
     -------
-    list or dict
+    list, dict, pandas Series, or pandas DataFrame
         Expanded events.
 
     """
@@ -517,24 +560,59 @@ def expand_events(
     if not isinstance(right_expand, pd.Timedelta):
         right_expand = pd.Timedelta(right_expand)
 
-    if isinstance(lists, list):
-        expanded = (
+    if isinstance(events, pd.Series):
+        labels = validate_series(events)  # type: pd.Series
+        lists = to_events(
+            labels, freq_as_period=freq_as_period
+        )  # type:List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]
+        expanded_lists = expand_events(  # type:ignore
+            events=lists, left_expand=left_expand, right_expand=right_expand
+        )  # type:List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]
+        expanded_labels = to_labels(
+            lists=expanded_lists,
+            time_index=labels.index,
+            freq_as_period=freq_as_period,
+        )  # type: pd.Series
+        expanded_labels.loc[
+            (expanded_labels == False) & (labels.isna())
+        ] = float("nan")
+        expanded_labels = expanded_labels.rename(labels.name)
+        expanded_labels.index = labels.index
+        return expanded_labels
+    elif isinstance(events, pd.DataFrame):
+        expanded_df = pd.concat(
+            [
+                expand_events(
+                    s,
+                    left_expand=left_expand,
+                    right_expand=right_expand,
+                    freq_as_period=freq_as_period,
+                )
+                for _, s in events.iteritems()
+            ],
+            axis=1,
+        )  # type: pd.DataFrame
+        return expanded_df
+    elif isinstance(events, list):
+        expanded_list = (
             []
         )  # type: List[Union[Tuple[pd.Timestamp, pd.Timestamp], pd.Timestamp]]
-        for ano in lists:
+        for ano in events:
             if isinstance(ano, tuple):
-                expanded.append((ano[0] - left_expand, ano[1] + right_expand))
+                expanded_list.append(
+                    (ano[0] - left_expand, ano[1] + right_expand)
+                )
             else:
-                expanded.append((ano - left_expand, ano + right_expand))
-        expanded = validate_events(expanded)
-        return expanded
-    elif isinstance(lists, dict):
+                expanded_list.append((ano - left_expand, ano + right_expand))
+        expanded_list = validate_events(expanded_list)
+        return expanded_list
+    elif isinstance(events, dict):
         return {
             key: expand_events(value, left_expand, right_expand)
-            for key, value in lists.items()
+            for key, value in events.items()
         }
     else:
-        raise TypeError("Arugment `lists` must be a list or a dict of lists.")
+        raise TypeError("Arugment `events` must be a list or a dict of lists.")
 
 
 def split_train_test(
diff --git a/src/adtk/visualization/_visualization.py b/src/adtk/visualization/_visualization.py
index b155348..92c0cce 100644
--- a/src/adtk/visualization/_visualization.py
+++ b/src/adtk/visualization/_visualization.py
@@ -167,12 +167,16 @@ def plot(
         Default: 4.
 
     freq_as_period: bool, optional
-        Whether to treat time stamps following regular frequency as time
-        spans. E.g. time index [2010-01-01, 2010-02-01, 2010-03-01, 2010-04-01,
-        2010-05-01] follows monthly frequency, and each time stamp represents
-        that month if freq_as_period is True. Otherwsie, each time stamp
-        represents the time point 00:00:00 on the first day of that month. This
-        is only used when anomaly given as binary series.
+        Whether to regard time index with regular frequency (i.e. attribute
+        `freq` of time index is not None) as time intervals. Only used when
+        anomaly is given as binary series.
+
+        For example, DatetimeIndex(['2017-01-01', '2017-01-02', '2017-01-03',
+        '2017-01-04', '2017-01-05'], dtype='datetime64[ns]', freq='D') has
+        daily frequency. If freq_as_period=True, each time point in the index
+        represents that day (24 hours). Otherwsie, each time point represents
+        the instantaneous time instance of 00:00:00 on that day.
+
         Default: True.
 
     axes: matplotlib Axes object, or array of Axes objects, optional
diff --git a/tests/test_expand_events.py b/tests/test_expand_events.py
index efeeab5..11cdcc8 100644
--- a/tests/test_expand_events.py
+++ b/tests/test_expand_events.py
@@ -2,7 +2,7 @@
 
 from adtk.data import expand_events
 
-events = [
+event_list = [
     pd.Timestamp("2017-1-1 20:04:00"),
     (pd.Timestamp("2017-1-1 20:00:00"), pd.Timestamp("2017-1-1 20:05:59")),
     (pd.Timestamp("2017-1-1 20:03:00"), pd.Timestamp("2017-1-1 20:08:59")),
@@ -12,10 +12,85 @@
     pd.Timestamp("2017-1-1 21:03:00"),
 ]
 
+nan = float("nan")
+event_labels = pd.Series(
+    [0, 0, 1, 1, nan, 0, 1, 0, nan, 0, 0, 1],
+    index=pd.date_range(start="2017-1-1", periods=12, freq="D"),
+)
+
+
+def test_expand_event_series_freq():
+    expanded_events = expand_events(
+        event_labels,
+        left_expand="1hour",
+        right_expand="1hour",
+        freq_as_period=True,
+    )
+    true_expanded_events = pd.Series(
+        [0, 1, 1, 1, 1, 1, 1, 1, nan, 0, 1, 1],
+        index=pd.date_range(start="2017-1-1", periods=12, freq="D"),
+    )
+    pd.testing.assert_series_equal(
+        true_expanded_events, expanded_events, check_dtype=False
+    )
+
+
+def test_expand_event_series_no_freq():
+    expanded_events = expand_events(
+        event_labels,
+        left_expand="1hour",
+        right_expand="1hour",
+        freq_as_period=False,
+    )
+    pd.testing.assert_series_equal(
+        event_labels, expanded_events, check_dtype=False
+    )
+
+
+def test_expand_event_df_freq():
+    expanded_events = expand_events(
+        pd.concat(
+            [event_labels.rename("A"), event_labels.rename("B")], axis=1
+        ),
+        left_expand="1hour",
+        right_expand="1hour",
+        freq_as_period=True,
+    )
+    true_expanded_events = pd.Series(
+        [0, 1, 1, 1, 1, 1, 1, 1, nan, 0, 1, 1],
+        index=pd.date_range(start="2017-1-1", periods=12, freq="D"),
+    )
+    true_expanded_events = pd.concat(
+        [true_expanded_events.rename("A"), true_expanded_events.rename("B")],
+        axis=1,
+    )
+    pd.testing.assert_frame_equal(
+        true_expanded_events, expanded_events, check_dtype=False
+    )
+
+
+def test_expand_event_df_no_freq():
+    expanded_events = expand_events(
+        pd.concat(
+            [event_labels.rename("A"), event_labels.rename("B")], axis=1
+        ),
+        left_expand="1hour",
+        right_expand="1hour",
+        freq_as_period=False,
+    )
+
+    pd.testing.assert_frame_equal(
+        pd.concat(
+            [event_labels.rename("A"), event_labels.rename("B")], axis=1
+        ),
+        expanded_events,
+        check_dtype=False,
+    )
+
 
 def test_expand_event_list():
     expanded_events = expand_events(
-        events, left_expand="1min", right_expand="3min"
+        event_list, left_expand="1min", right_expand="3min"
     )
     assert expanded_events == [
         (pd.Timestamp("2017-1-1 19:59:00"), pd.Timestamp("2017-1-1 20:11:59")),
@@ -26,7 +101,9 @@ def test_expand_event_list():
 
 def test_expand_event_dict():
     expanded_events = expand_events(
-        {"A": events, "B": events}, left_expand="1min", right_expand="3min"
+        {"A": event_list, "B": event_list},
+        left_expand="1min",
+        right_expand="3min",
     )
     assert expanded_events == {
         "A": [