diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -11,7 +11,7 @@
 import contextlib
 
 from . import result
-from .util import (strclass, safe_repr, _count_diff_all_purpose,
+from .util import (strclass, safe_repr, truncate_str, _count_diff_all_purpose,
                    _count_diff_hashable, _common_shorten_repr)
 
 __unittest = True
@@ -770,7 +770,15 @@
     def _baseAssertEqual(self, first, second, msg=None):
         """The default assertEqual implementation, not type specific."""
         if not first == second:
-            standardMsg = '%s != %s' % _common_shorten_repr(first, second)
+            firstrepr, secondrepr = _common_shorten_repr(first, second)
+            # check that standardMsg will be shorter than maxDiff
+            maxDiff = self.maxDiff
+            if (maxDiff is not None and
+                    (len(firstrepr) + len(secondrepr) + 8) > maxDiff):
+                standardMsg = '%s != %s' % (truncate_str(firstrepr, maxDiff),
+                                            truncate_str(secondrepr, maxDiff))
+            else:
+                standardMsg = '%s != %s' % (firstrepr, secondrepr)
             msg = self._formatMessage(msg, standardMsg)
             raise self.failureException(msg)
 
diff --git a/Lib/unittest/util.py b/Lib/unittest/util.py
--- a/Lib/unittest/util.py
+++ b/Lib/unittest/util.py
@@ -49,7 +49,10 @@
         result = object.__repr__(obj)
     if not short or len(result) < _MAX_LENGTH:
         return result
-    return result[:_MAX_LENGTH] + ' [truncated]...'
+    return truncate_str(result)
+
+def truncate_str(obj, maxlen=_MAX_LENGTH):
+    return obj[:maxlen] + ' [truncated]...'
 
 def strclass(cls):
     return "%s.%s" % (cls.__module__, cls.__name__)