Objective To evaluate the mechanism of the mesial impaction of mandibular third molars（M3） in adults and the relationship between the mesial impaction or eruption of M3 and the development of maxillary and mandible. Methods 72 subjects who have taken panoramic radiographs and lateral cephalometric radiographs before orthodontic treatments were divided into two groups based on the status of M3: Impaction group and eruption group, and all cephalometric radiographs were analyzed for the two groups. Results Mandibular body length（Go- Po’）, ramal width（W）, mandibular plane angel（MP-FH）, lower anterior face height（ANS-Me） and ramal length/ mandibular length ratio（Co-Go/Co-Po） were significantly difference（P0.05） and Go-Po’, ANS-Me, MP-FH were decreased in the impaction group. The impaction and eruption groups did not differ significantly（P
