The microstructural changes in the initial stage of a conversion process of α-tricalcium phosphate [α-Ca3(PO4)2] (α-TCP) to hydroxyapatite [Ca10(PO4)6(OH)2] (HAp) by the hydrolysis method were investigated by transmission electron microscopy (TEM). To investigate the microstructural changes that take place during the conversion process, we prepared two types of α-TCP specimens for TEM: α-TCP powder and sintered α-TCP thin film. According to our results, the microstructural changes can be summarized as follows. At first, the surface of the α-TCP was covered with an amorphous calcium phosphate layer, resulting from hydration or the dissolution of α-TCP. Subsequently, the nucleation of HAp occurred on the amorphous layer, and then dendritic structures appeared on the layer. Thereafter, the dendritic structures would grow into needlelike fine HAp crystals.