The purpose of this study was to obtain some informations for the radiographic differential diagnosis between odontogenic keratocyst and unicystic ameloblastoma in the mandible. The author compared and analysed the clinico-radiographic features of 48 cases of odontogenic keratocyst and 32 cases of unicystic ameloblastoma. The obtained results were as follows : 1. Odontogenic keratocyst and unicystic ameloblastoma occurred the most frequently in the 2nd and 3rd decades, and both lesions occurred with slight predilection in males. The most frequent lesional site was molar area in odontogenic keratocyst(50.0％) and mandibular angle and ramus area in unicystic amelobla-stoma(71.9％). 2. Cortical thinning and expansion were observed with similar occurrences in odontogenic keratocyst(77.l％) and in unicystic ameloblastoma(72.9％). 3. Typical undulating lesional border was observed more frequently in odontogenic keratocyst(79.2％) than in unicystic ameloblastoma(46.9％). 4. Well-defined lesional outline occurred more frequently in odontogenic keratocyst(97.9％) than in unicystic ameloblastoma(53.1％). 5. Root resorption of adjacent teeth occurred more frequently in unicystic ameloblastoma(65.2％) than in odontogenic keratocyst(18.8％) respectively, but loss of lamina dura was frequently observed in odontogenic keratocyst(79.2％). And tooth displacement occurred more frequently in odontogenic keratocyst(50.0％) than in unicystic ameloblastoma(17.4％). 6. Displacement of mandibular canal occurred more frequently in odontogenic keratocyst(75.0％) than in unicystic ameloblastoma(61.5％). 7. Inhomogeneous lesional radiolucency occurred more frequently in unicystic ameloblastoma(53.l％) than in odontogenic keratocyst(39.6％).