A kind of Chinese herb medicine--Wumingyi chongji could promote fracture healing had been proved. In order to disclose the mechanism, a standard fracture model was produced in 50 healthy male Newzealand rabbits. The animals were randomly divided into experimental group and control group. Another 5 rabbits without operation act as nomal. By immunohistochemical study, the bone morphogenetic protein (BMP) content in callus was calculated by computerized interacive morphometry (CIM) in the 1st, 2nd, 3rd, 4th, 5th week after fracture. The results were: BMP in callus was much more than in nomal (P lt; 0.01); BMP in callus increased from the 1st week to the 3rd week, decreased from 4th week to 5th week, and was more in experimental group in the 1st and 2nd week than in the control group (P lt; 0.01). The highest BMP content appeared in the 2nd week in the experimental group, while in the 3rd week in the control group. It was concluded that Wumingyi chongji could promote osteoblast to synthetize BMP, BMP induce preosteoblast change into osteoblast. Thus the rate of fracture healing could be increased.