Abstract: Objective：To study the change of alveolar bone height and width after tooth extraction by using CBCT. Methods：42 patients with 68 teeth extraction at outpatient department were selected randomly. CBCT was taken preoperatively and 6 months after extraction. The height and width of alveolar ridge before extraction and 6 months later were measured and the width of alveolar ridge were measured at 6 planes, then the absorption of the posterior alveolar bone in the extraction of the posterior teeth was analyzed. Results：Six months after the extraction of the posterior teeth, the alveolar bone height decreased by (1.15±0.46) mm. The alveolar height decreased by (1.83±1.87) mm in the buccal and decreased by (0.99±0.50) mm in the lingual. The average decrease of the male alveolar ridge height was (1.29±0.31) mm, and that of the female alveolar ridge height was (0.69±0.27) mm 6 months after the extraction of the posterior teeth. There were significant difference in the decreased heights of alveolar bone between the buccal and the lingual, male and female. The average decrease of the male alveolar ridge width were (1.93±1.55), (1.34±1.23), (0.98±0.97), (0.45±0.53), (0.19±0.32), (0.06±0.17) mm at 6 planes, respectively.The horizontal absorption of the alveolar ridge in each plane was different . Conclusions： Six months after the extraction of the posterior teeth, the alveolar bone absorbs significantly in the horizontal and vertical, the buccal bone wall absorbs more than the lingual side, and the male absorbs more in the vertical bone than the female. The bone absorption is most at the top of the alveolar ridge, followed by the middle, and basically no absorption at the base.