Objective To investigate the development of third molars of twelve years old teenagers in Chengdu through X -ray films. Methods Randomly selected 449 panoramic radiographs of twelve -year -olds from the database of West China Hospital of Stomatology, Sichuan University, and divided the development degree of third molars into several phrases to analysed the development status according to different genders, the left and right sides, the maxilla and the mandible. Results Most part of the crowns of the third molars of the twelve years old teenagers were mineralized. There was no significant differences between male and female about the development degree of third molars. Mostly, the development of third molars appeared earlier in the mandible than that in the maxillary. The occurrence ratio was 60.69% on the left side and 64.25% on the right side. The abnormal development direction of third molar in the maxillary appeared vertical tilting and in the mandible appeared mesial tilting more frequently. Conclusion The crowns of the twelve-year-olds’third molars were mostly mineralized. The mandible third molars appeared earlier than maxillary ones.
