The effect of hydrothermal reaction temperature and pH on crystallite size, phase stability, and sintered density of hydroxyapatite (HAp) has been investigated. The crystallite size increased with the increase in temperature but was not significantly affected by pH. The decomposition of HAp into tricalcium phosphate was observed at 1250°C and the extent of decomposition decreased with increase in pH. The influence of temperature and pH on the crystallite size and the decomposition was related to the decrease in solubility of Ca2+. The phase stability of HAp, prepared hydrothermally, in HAp/zirconia composites was higher than that of commercial HAp due to its high sintered density.