In this study, hydroxyapatite (HAp) nanopowder (Ca10(PO4)2 (OH)6) were synthesized using calcium nitrate tetrahydrate Ca(NO3)2.4H2O and diammonium hydrogen phosphate (NH4)2HPO4 by hydrothermal method with different stoichiometric ratios of Ca and P. The effect of hydrothermal temperature, time, and pH on the Ca/P ratio and thermal stability of HAp samples were investigated. Experimental results have shown that a pure HAp phase with a stoichiometric ratio (1.676) was observed in alkaline conditions. This study showed that with an increase in hydrothermal temperature and time, the pure HAp phase formation increases. The Ca/P ratio increases and gets close to a stoichiometric ratio, and thermal stability also increases. The study revealed that the highest thermal stability was found in stoichiometric HAp samples prepared in alkaline conditions.