Abstract: Objective 　To investigate the changes of proliferating cell nuclear antigen(PCNA) in rats' periodontal ligament in orthodontic tooth movement and the mechanism of changes of PCNA during orthodontic tooth movement. Methods Young SD rats were used to build up orthodontic movement models. The animals were sacrificed 24 hours,3 days,5 days,7 days,14 days and 21 days after continuous mechanical stress was applied. The expressions of PCNA were performed by immunohistochemistry. Results Expression of PCNA in rats' periodontal ligament was weak in the control group. In the experimental group, the expression of PCNA on the tension side of periodontal ligament got stronger 24 hours after continuous mechanical stress, strongest 3 and 5 days after continuous mechanical stress, weaker 7 days after the stress, and normal 21 days after the stress. Conclusion 　Orthodontic treatment promotes cell proliferation in periodontium, and completes reconstruction of periodontium.