Dry grinding of Ca(OH)(2)-P2O5 and CaO-Ca(OH)(2)-P2O5 mixtures was conducted by a planetary ball mill to investigate the mechanochemical solid-phase reaction for the synthesis of hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAp). HAp was synthesized by grinding of the two sets of mixtures. The formation of HAp from the Ca(OH)(2)-P2O5 mixture was more advantageous than that from the CaO-Ca(OH)(2)-P2O5 one. This synthesis reaction from the former mixture was almost completed within 30 min of grinding. The presence and amount of H2O contained in the starting mixtures played a key role to promote the formation of HAp. Especially, in the former mixture, the prolonged grinding assisted the solid-phase reaction of the intermediate DCPD and Ca(OH)(2) to produce HAp more effectively.