PURPOSE: To investigate the effect of defatted safflower seed or matairesinol on spontaneous bone formation in surgically induced bone defects in young rabbits MATERIALS AND METHODS: Bone defects(20% of original femoral length) were created at the midshaft of the femur in 52 young rabbits and stabilized with external fixation. The periosteum was preserved. Fifty-two rabbits were divided into four groups as follows; the group I fed on safflower seed powders(30%) and normal diet(70%), the group II on matairesinol (5%) and normal diet(95%), the group III on normal diet(100%) with intravenous injection of matairesinol, the group IV on normal diet(100%). Radiographs were taken weekly to evaluate the bone regeneration and union time. Biomechanical testing on the new bone formation area was performed to measure bending stiffness. The area of new bone formation was scanned by quantitative computed tomography to measure bone mineral density(BMD). RESULT: The mean union time(weeks) was 7.2 in the group I, 8.6 in the group II, 8.8 in the group III, and 8.5 in the group IV and was significantly different between the group I and the other groups(p<0.05). The BMD and bone strength of the callus were compared between the group I and the group IV. The mean BMD was 310.45 ±49.58 g/mm2 in the group I and 291.16 ±55.79 g/mm2 in the group and there was significant difference(p<0.05). The mean bending stiffness was 415.33 ±137.18 N/mm in the group I and 358.75 ±107.32 N/mm in the group IV and there was significant difference(p<0.05). CONCLUSION: The diet with safflower seed powder was effective for decreasing union time and increasing bone strength of the callus formed at the bone defect whereas the diet or injection with matairesinol was not effective. More experimental studies are necessary to prove the effect of matairesinol on the callus.