Mechanochemical solid-phase reactions for hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAp) from different kinds of constituent powder mixture were investigated. HAp monophase can he produced from the three sets of powder mixtures of Ca(H2PO4)(2)H2O-CaO, Ca(H2PO4)(2)H2O-Ca(OH)(2), and Ca(H2PO4)(2)H2O-CaCO3 (Ca/P atomic ratio: 1.67) by dry grinding using a planetary ball mill at room temperature. All the solid-phase reactions for HAp from the starting mixtures were almost completed within 60 min of grinding. Among them, Ca(H2PO4)(2)H2O-Ca(OH)(2) mixture is the most suitable one for the preparation of HAp. The amount of free H2O released during mechanochemical treatment plays a key role in these synthesis reactions. The formation of HAp is improved as grinding progresses. The completion of synthesis reactions is confirmed by measuring the equilibrium pH of powder suspensions of ground mixtures.