Bone regeneration by calcium phosphates has been known to be intricately dependent on material properties or implanted milieu of host animals, such as site and species. Critical sized calvarial defects of mouse were recently used as the model for investigating bone regeneration ability and the mechanisms. The purpose of the present study is to investigate whether the critical sized mouse calvarial defects can be utilized to examine bone regeneration with synthetic octacalcium phosphate (OCP). OCP , prepared by wet synthesis methods, was sieved 0.3 ~ 0.5 mm in diameter and used for the animal experiment. At 14 days after surgery, histological examination showed that implantation of OCP grafted defects significantly enhanced bone formation compared with the control defect. OCP tended to convert to hydroxyapatite with time. The tartrate-resistant acid phosphatase (TRAP) positive osteoclastic cells were observed around the OCP particles. The results suggest that the mouse critical sized calvarial bone defects are useful model to investigate the bone formation by the OCP implantation.