The location of the mandibular canal and the cortical thickness of the mandible is important in the practice of dentistry. This study was performed on twenty chosen dry mandibles, which were of adults and included fully erupted premolars and molars. The purpose of this study was to evaluate the location of the mandibular canal and the cortical thickness of the mandible on computed tomograms and to aid in the surgical treatment plans. The obtained results were as follows; 1. The horizontal distance between the mandibular canal and the buccal external border was 6.6±0.9mm on Somesial root of the first molar), and it was increased posteriorly. The horiwntal distance between the mandibular canal and the lingual external border was 4.1±1.lmm on S₀, and it was decreased posteriorly. 2. The vertical distance between the alveolar crest and the mandibular canal was 16.9±1.6mm on S₀, and it was decreased posteriorly. The vertical distance between the inferior border of mandible and the mandibular canal was 8.8±1.3mm on S₀, and it was increased anteriorly and posteriorly. 3. The thickness of the buccal cortical plate was 2.2±0.4mm on S₀. and it was increased posteriorly. But, that of the lingual cortical plate was 2.0±0.6mm on S₀ and it was decreased posteriorly. 4. The area of the buccal cortical plate was 66.5±1.0mm² on S₀. and it was increased posteriorly. But, that of the lingual cortical plate was 65.8±0.9mm² on S₀ and it was decreased posteriorly.