Abstract: Objective To evaluate the thickness of buccal cortical bone and bone depth at different heights in posterior area of mandible where mini-plants were placed．Methods Sixty-eight cone-beam CT（CBCT） images were selected randomly from the Affiliated Stomatology Hospital，China Medical University, which consisted of 36 male and 32 female ranging from 18 to 31 years old. Thickness of cortical bone between the first premolar distal and the second molar mesial at the height of 3，6，and 9 mm from the alveolar crest was determined. The data were analyzed statistically. Results There was no significant difference in the cortical bone thickness and bone depth between the left and right side（P ＞ 0.05）.And no significant difference was found between males and females（P ＞ 0.05）. The thickness of cortical bone and bone depth between the first molar and the second molar in mandible was maximum at 3 heights, between the first premolar and the second premolar in mandible the thickness and bone depth was the minimum at 3 heights. Conclusion The cortical bone thickness in the buccal posterior region and bone depth of mandible change along with the position changing sagittally and vertically，which should be considered in mini-plants implantation. A position as far from the alveolar crest as possible and near the distal end of the arch is the best in posterior area of mandible. Cortical bone thickness and bone depth of mandible are, on average, favorable for the insertion of orthodontic mini-implants.

Key words: cortical bone thickness, bone depth, micro implant anchorage, cone-beam CT（CBCT）