Hydroxyapatite (HA) was synthesized by the hydrothermal treatment of octacalcium phosphate (OCP). Powder of OCP was hydrothermally treated with distilled water at 120-240°C for 3-24 h. Only diffraction lines assigned to HA were detected by X-ray diffraction (XRD) even after the treatment at 120°C for 3 h. The crystallinity of the formed HA increased with increasing the treatment temperature and period. Plate-shaped particles were split into small strip-shaped particles by the hydrothermal treatment and this phenomenon was obvious when the treatment temperature was high and the period was long. However, the plate-shape morphology was remained when OCP was transformed to HA at 120°C for 3 h. The hydrothermal treatment of OCP is useful to synthesize the plate-shaped HA particles in a short period.