Ameloblastoma is one of the well-known odontogenic tumours that can be associated with calcifying odontogenic cysts (COCs), but only a few reports include detailed clinical and radiographic features. In this paper we demonstrate a case of large ameloblastomatous COC in the mandible. The radiographic examination revealed the presence of a multilocular cystic lesion in the right posterior part of the mandible containing the impacted lower second molar with remarkable expansion toward both lingual and buccal side. This feature was different from the general findings of COC and rather resembled that of ameloblastomas.