The osteochondroma is the most common benign bone tumor, of which the typical form is a cartilage-capped bony projection, far often found at the metaphysis of the long tubular bones, and may arise from any bone preformed in cartilage. To our knowledge, however, osteochondroma of accessory bones has not been reported, but we experienced a rare case of osteochondroma located in the position normally occupied by a tarsal accessory bone(Os Tibiale Externum). The mass was treated by excisioa and it discloeed typical microscopic findings of ostechondroma as shown in the pictures.