Statement of Purpose: We have reported that octacalcium phosphate (OCP) exhibits direct bone bonding property1 and has in vitro stimulatory capacities on osteoblastic cell differentiation from bone marrow cells not only to osteoblastic cells2,3 but also to late osteocytes4. OCP is also able to enhance formation of osteoclasts from bone marrow cells in the co-presence of osteoblasts5. From these intrinsic mechanisms, OCP is considered to enhance new bone formation coupled with biodegradation6. We hypothesized, therefore, that OCP could be used as a bone substitute material to treat bone defect in pathologic condition such as osteoporotic condition by expressing such cellar stimulatory activities. In the present study, an effect of OCP/gelatin (OCP/Gel) bone substitute material7 on bone formation in a tibial bone defect was investigated using an ovariectomized rat. OCP/Gel has shown to have highly biodegradable characteristics with an immediate bone deposition in a tibial bone defect of a rabbit8. The resorption rate was much higher in OCP/Gel than a high porosity β-tricalcium phosphate (β-TCP).