The hematoma in the fracture site after injury plays a role of immobilization of the fracture and forms a framework for the infiltration of cells and the ingrowth of vessels. It is well-known that the hematoma is one of the most important factors in the process of fracture healing. However, it is not clear why the hematoma is important and how it plays a role during the fracture healing. The purpose of this paper is to show the ability of bone formation in the hematoma itself. An experimental fractures were made at the midshaft of the tibia in rats. After removal of the hematoma at the fracture site, the hematoma was transplanted surgically into the subperiosteal area in the skull of these rats. After sacrificing them, histological observation of the hematoma was done at 1, 2 and 3 weeks after the fracture was made. As controls, the following three groups whose parietal periosteum was ablated in the same manner, were used 1) rats that had nothing implanted, 2) rats which were transfused by peripheral blood from the fresh wound of the tail skin, 3) rats with transplantation of bone marrow from the femur. In the group of rats with the transplanted hematoma 2 days after the fracture, 11 of 35 cases (31%) had new bone formation, it was found in 8 of 19 cases (42%) in the group of rats with the transplanted hematoma 4 days after the fracture. New bone formation was observed in 4 of 10 rats (40%) with transplanted bone marrow. The rats with only periosteal ablation or peripheral blood transplantation, however, did not form new bone beneath the periosteum. In conclusion, the hematoma in the fracture site has osteogenic potential and plays an important role in the process of fracture healing.