Objective: To understand the effects of rectangle-shaped intramedullary nailing on the bone callus and the bone morphogenetic protein content in osteoblast. Methods: Twenty-four New Zealand rabbits tibial fractures were internally fixed by RIM and plates, animals were killed in groups successively at 14, 21, and 28 d and samples from bone callus of the fracture site were reacted with BMP-Ab by immunochemistry method. And the results were quantified by IAS. X-ray films were taken of both tibia 56 d after operation, and the X-ray films were also quantified by IAS. Results: BMP content in single cell from flexible fixation group is significantly higher than that of plating group, and bone callus content from the RIM fixation group is also richer than that of the plating group. Conclusion: After RIM internal fixation, there is micromovement between the bone fragments, and this may stimulates the secretion of BMP. And the increment of BMP secretion may lead to richer bone callus formation.