A case of an unicystic ameloblastoma of the mandible presenting as an apical periodontal cyst was reported. The lesion showed an unilocular radiolucency with well delineated border. Histologic examination revealed that a fibrous cyst wall with a lining that consists of partially of ameloblastic epithelium. The overlying epithelial cells are loosely cohesive and resemble stellate reticulum. The fibrous cyst wall of the cyst is infiltrated by typical follicular ameloblastoma. but devoid of inflammatory reaction in the cystic wall. Some considerations regarding differential diagnosis. pathogenesis and biologic behaviour of the lesion were discussed.