diff --git a/vobject/icalendar.py b/vobject/icalendar.py
index 6ce5502..a82eac0 100644
--- a/vobject/icalendar.py
+++ b/vobject/icalendar.py
@@ -899,13 +899,19 @@ def transformFromNative(obj):
             obj.value_param = "DATE"
             obj.value = ",".join([dateToString(val) for val in obj.value])
             return obj
-        # Fixme: handle PERIOD case
         else:
             if obj.isNative:
                 obj.isNative = False
                 transformed = []
                 tzid = None
                 for val in obj.value:
+                    if obj.value_param == "PERIOD":
+                        if type(val[0]) is datetime.datetime and type(val[1]) is datetime.timedelta:
+                            transformed.append(periodToString(val))
+                            continue
+                        if type(val[0]) is datetime.datetime and type(val[1]) is datetime.datetime:
+                            transformed.append(dateTimeToString(val[0]) + '/' + dateTimeToString(val[1]))
+                            continue
                     if tzid is None and type(val) is datetime.datetime:
                         tzid = TimezoneComponent.registerTzinfo(val.tzinfo)
                         if tzid is not None: