The conversion of brushite (CaHPO2 · 2H2O; DCPD) into hydroxyapatite (HAp) by hydrolysis has been studied by separating the conversion process into two stages, i.e. the structural change of DCPD into HAp (1) and the subsequent compositional increase in Ca/P ratio of the HAp (II). In Reaction I at 40°C, HAp formed most rapidly at around pH 7.5 to 8.0. The complete conversion was observed within 2.5 h at 40°C, 1 h at 60°C and only 5 min at 80°C. The compositions of HAp thus formed were nonstoichiometric and had a Ca/P ratio below 1.60. It was difficult to increase the Ca/P ratio up to the stoichiometric value 1.67, because the adjustment of pH to higher values and/or the addition of Ca2+ ions to accelerate the increase in Ca/P ratio, retarded the proceeding of Reaction I. On the other hand, in Reaction II, such pH adjustment and Ca2+ addition were remarkably effective in increasing the Ca/P ratio. Consequently, two-stage processing was reasonable and convenient for the preparation of stoichiometric HAp, because it was possible to manage the controlling factors in both Reactions I and II independently. The resulting HAp powders showed a comparatively low crystallinity similar to precipitated HAp and large weight losses (above 6%) on heating, and were composed of dense aggregates of irregular thin microcrystals.