Duplication of the jejunum in a dog was morphologically reported. A solitary lesion with a diameter of 4 mm was incidentally found near the mesenteric side of the intestinal wall. On the intestinal mucosa, a crater-like focus was observed. Histologically the nodular lesion was detected as a round and clearly circumscribed canal within the normal muscular layer of the small intestine. The inner surface of the canal had mucosa which was similar to the normal intestinal mucosa with villi in light and electron microscopy.