The mandibular canal must be considered carefully during surgical treatment, especially surgical extraction of the impacted tooth and intraosseous implant because it contains the important inferior alveolar nerve and vessels. The author investigated the curvatUre of the mandibular canal, the positional frequency of mandibular foramen to the occlusal plane and gonial angle and the positional frequency of the mental foramen to the tooth site using orthopantomograms. The materials consisted of 295 orthopantomograms divided into seven groups ranging from the first decade to 6th. decade. The results were as follows: 1. The position of mandibular foramen was most frequently below occlusal plane in Group Ⅰ (78.6%) and Group Ⅱ (71.2%), above occlusal plane in Group Ⅲ (63.0%), Group IV (71.1%), Group V (57.6%), Group (76.7%) and Group VII (70.0%). 2. The curvature of mandibular canal was 142.8° in Group Ⅰ, 142.09° in Group Ⅱ, 139.34° in Group Ⅲ, 141.48° in Group Ⅳ, 138.45° in Group Ⅴ, 140.77° in Group Ⅵ and 143.89° in Group Ⅶ. 3. The gonial angie was 125.82° in Group Ⅰ, 123.18° in Group Ⅱ, 124.06° in Group Ⅲ, 120.45° in Group Ⅳ, 121.12° in Group Ⅴ, 121.63° in Group Ⅵ and 121.24° in Group Ⅶ. 4. The position of the menta] foramen was most frequently below the apex of mandibular first premolar in Group Ⅰ (57.2%), between the apex of mandibular first and second premolar in Group Ⅱ (59.6%) and Group Ⅲ (48.9%), and below the apex of mandibular second premolar in Group Ⅳ (39.2%), Group Ⅴ (48.5%) Group Ⅵ(46.6%) and Group Ⅶ(56.4%)