Octacalcium phosphate (OCP) is regarded as an in vivo precursor of hydroxyapatite (HA). It is important to understand the mechanism of transformation of OCP to HA in order to reveal the mechanism of mineralization and help in the development of artificial bone-repairing materials. Herein, we have examined the behavior of OCP in a simulated body fluid (SBF) and pure water. The OCP particles immersed in the SBF at 37 °C did not transform to HA even after 720 h of immersion, though the particles showed crystal growth. In distilled water at 60 °C, the OCP particles transformed to HA but the unreactive period was observed. Although the immersed solution became supersaturated with HA within 12h of immersion, the OCP was not transformed in the first 36 h of immersion. These results indicate that the nucleation of HA is the rate-determining step in the transformation of OCP to HA.