The osteoinductivity of calcium phosphate ceramics has been studied extensively, but the mechanism is still unclear and few reports about the molecular mechanism in the osteoinductive process. In this study the osteoblast related gene expressions induced by biomaterials were investigated by isolating the RNA from the tissue grown in porous hydroxyapatite/tricalcium phosphate (HA/TCP) ceramics implanted in rat femur muscle on day 7, 15, 30, 60, 90,120, and analyzed by RT-PCR technique. RNA extracted from muscle without implant was used as control at the same time. The results showed that osteopontin and osteocalcin genes, the important osteoblastic markers, expressed in early stage, on day 7 after implantation, and were detected at any period. Collagen type I gene expressed on day 60, 90 and 120. It revealed that osteoblast differentiation occurred very early before collagen type I expression after implanting HA/TCP ceramics in vivo.