The formation, maintenance, and regeneration of bone is a complex precess involving the interactions of many cellular elements with systemic and local regulators. TGF-β is one of growth factors that play an important role in the formation and remodeling of bone. In vitro studies have suggested that TGF-β regulates chondrogenesis and possibly osteogenesis by affecting replication, gene expression, and structural protein synthesis in bone formation. We investigated the effect of TGF-β1 upon fracture callus formation and maturation in mature rate. Closed femoral shaft fracture was made consistently by three point stress technique after percutaneous intramedullary nailing. TGF-β1 was injected subperiosteally at the fracture site daily for 2 weeks. We examined the effect of TGF-β1 on the fracture healing process with the radiographic, densitometric, histologic, and immunohistochemical methods. The following results were obtained. 1. Radiographic examination demonstrated that TGF-β1 injection group appeared to have more abundant callus formation and earlier callus maturation as compared to the control group. 2. Bone densitometric examination revealed that TGF-β1 injection group had higher bone mineral density and content that the control group. 3. Thermographic examination revealed that TGF-β1 injection group had higher local temperature at the injection area than the control group. 4. Histologic examination suggested that TGF-β1 stimulates and accelerates fracture callus formation and endochondral bone formation. 5. Immunohistochemical examination revealed that chondrocytes at the fracture site in the TGF-β1 injection group seemed to produce type I collagen.