Abstract: Objective: To explore the law of tooth movement and relapse induced by different orthodontic forces through in vivo Micro-CT dynamic observation. Methods: Fifteen 10-week-old rats were divided into three groups randomly. Force of 20g, 50g and 100g were delivered to the first molars of left maxillary for 14 days. The Micro-CT scan was taken at day 0, 3, 7,10,14,28 after the appliance was applied and day 3, 7,10,14,28,42 after the appliance was removed.Thedistance oftooth movement at different time points was measured. Results: At first 3 days, the first molars of all groups mesially moved significantly. From day 3 to day 10, the tooth movement became slower. The 20g and 100g groups moved rapidly after day 10.In the 20g groups, the tooth movement relapsed significantly during day 0 to day 3, the 50g and 100g groups relapsed significantly during day 0 to day 7. At day 28, all groups almost completely relapsed. Conclusion: The process of tooth movement showed the pattern of an instantaneous displacement, delay, and a fast moving. The light force induced more tooth movement. The distance relapsed dramatically in the initial stage after the appliance was removed, and the greater the movement distance, the faster the relapse occurred.