Objective：To study the expression of bone morphogenetic protein (BMP)-2 during midfacial sutural distraction osteogenesis in dogs and to probe the mechanism of sutural distraction osteogenesis. Methods: Eighteen mongrel dogs (12-week old) were randomly divided into distraction osteogenesis group(n=12) and the control group(n=6).Rigid external distractor was used for sutural distraction of the midfacial skeleton in the experimental animals. Two experimental and one control dogs were sacrificed at defined time points (on the 5th, 10th and 15th day of distraction, the 10th and 20th day of fixation, and 30th day after completion of fixation).The expression of BMP-2 was observed by means of immunohistochemistry. Results: During the period of distraction and fixation, positive staining of BMP-2 was found in the proliferated preosteoblasts, the mesenchymal cells and the newly formed bone matrix at the sutural margin of the experimental dogs; weak expression was also noticed in the control dogs. Compared with that in the control group, the expression of BMP-2 in the experimental group increased and reached the peak on the 15th of distraction, then gradually decreased. the expression of BMP-2 nearly restored to the normal level 1 month after the removal of the distractor. Conclusion: The mechanical strain can lead to the generation of endogenous BMP-2, which may play an important role in the new bone formation.