The present study was designed to investigate whether hydrolysis of synthetic octacalcium phosphate (OCP) into hydroxyapatite affects bone formation. Mouse bone marrow stromal ST-2 cells and primary calvarial osteoblastic cells were cultured on the dishes pre-coated with OCP or its hydrolyzed Ca-deficient hydroxyapatite (OCP hydrolyzate; HL). The capacity of proliferation and differentiation was determined up to day 20. Granules of OCP and HL were implanted into critical-size rat calvaria defects for 4 and 12 weeks, and then bone formation was measured by histomorphometry. Structural changes of incubated and implanted OCP were determined by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The proliferation of both ST-2 and primary osteoblasts cultured on OCP or HL was initially inhibited, whereas their differentiation to osteoblasts was promoted at last. Implantation of OCP in bone defect more significantly enhanced bone formation than that of HL until 12 weeks. OCP tended to convert to apatite in vitro and in vivo. The conversion of the implanted OCP was ascertained to advance gradually with implantation periods. Taken together, these results suggest that OCP supports appositional bone formation and OCP-apatite conversion may be involved in this stimulatory capacity of OCP.