PURPOSE: To evaluate effect of growth hormone on osteoblast and new bone formation. MATERIALS AND METHODS: Bone defect of the tibia with preserved periosteum was made and fixed with external fixator. Intramuscular injection of growth hormone for 8 weeks in experimental group and saline in control group was performed. New bone formation at the bone defect in radiograph and bone mineral density (BMD) by quantitative computed tomography were evaluated at 8 weeks after surgery. Rat osteosarcom cells were cultured in both group to evaluate cell viability of osteoblast, alkaline phosphatase activity, and mRNA expression of osteocalcin by RT-PCR. RESULTS: Experimental group showed more callus formation and higher BMD at the bone defect site and the distal tibia compared to control group and there was significant difference. Proliferation of osteoblast, alkaline phosphatase activity, mRNA of osteocalcin at 5 days after culture were significantly higher in experimental group than those in control group. CONCLUSIONS: Growth hormone has positive effect on osteoblast and callus formation in vivo and vitro studies.