diff --git a/tools/releasetools/test_blockimgdiff.py b/tools/releasetools/test_blockimgdiff.py
new file mode 100644
index 0000000..03e8c8b
--- /dev/null
+++ b/tools/releasetools/test_blockimgdiff.py
@@ -0,0 +1,77 @@
+#
+# Copyright (C) 2016 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+from __future__ import print_function
+
+import common
+import unittest
+
+from collections import OrderedDict
+from blockimgdiff import BlockImageDiff, EmptyImage, DataImage, Transfer
+from rangelib import RangeSet
+
+class BlockImageDiffTest(unittest.TestCase):
+
+  def test_GenerateDigraphOrder(self):
+    """Make sure GenerateDigraph preserves the order.
+
+    t0: <0-5> => <...>
+    t1: <0-7> => <...>
+    t2: <0-4> => <...>
+    t3: <...> => <0-10>
+
+    t0, t1 and t2 must go before t3, i.e. t3.goes_after =
+    { t0:..., t1:..., t2:... }. But the order of t0-t2 must be preserved.
+    """
+
+    src = EmptyImage()
+    tgt = EmptyImage()
+    block_image_diff = BlockImageDiff(tgt, src)
+
+    transfers = block_image_diff.transfers
+    t0 = Transfer(
+        "t1", "t1", RangeSet("10-15"), RangeSet("0-5"), "move", transfers)
+    t1 = Transfer(
+        "t2", "t2", RangeSet("20-25"), RangeSet("0-7"), "move", transfers)
+    t2 = Transfer(
+        "t3", "t3", RangeSet("30-35"), RangeSet("0-4"), "move", transfers)
+    t3 = Transfer(
+        "t4", "t4", RangeSet("0-10"), RangeSet("40-50"), "move", transfers)
+
+    block_image_diff.GenerateDigraph()
+    t3_goes_after_copy = t3.goes_after.copy()
+
+    # Elements in the set must be in the transfer evaluation order.
+    elements = list(t3_goes_after_copy)
+    self.assertEqual(t0, elements[0])
+    self.assertEqual(t1, elements[1])
+    self.assertEqual(t2, elements[2])
+
+    # Now switch the order of t0, t1 and t2.
+    transfers[0], transfers[1], transfers[2] = (
+        transfers[2], transfers[0], transfers[1])
+    t3.goes_after.clear()
+    t3.goes_before.clear()
+    block_image_diff.GenerateDigraph()
+
+    # The goes_after must be different from last run.
+    self.assertNotEqual(t3_goes_after_copy, t3.goes_after)
+
+    # Assert that each element must agree with the transfer order.
+    elements = list(t3.goes_after)
+    self.assertEqual(t2, elements[0])
+    self.assertEqual(t0, elements[1])
+    self.assertEqual(t1, elements[2])