A 5-years-old 3.7 kg intact female Pomeranian dog was presented with the history of swelling and pain in the distal part of the right forelimb. The swollen lesion was firm as bony material on palpation. On radiographic examination, there was a well-marginated, radiolucent, and expansile bone lesion in the distal metaphysis of the right ulna. Because of very narrow zone of transition, well-defined margins and absence of any periosteal reactivity, benign bone cyst was strongly suspected. The lesion was removed by en bloc resection and packed the space with $Gelfoam^{(R)}$ and aneurysmal bone cyst was finally diagnosed according to histopathological examination. Normal gait was showed on postoperative day 9 and there have been no pain and complication for 1 year since then.