Hydroxyapatite(HAP) nanoparticles were synthesized by a mixed hydroxide method using calcium nitrate tetrahydrate[Ca(NO3)24H2O] and phosphoric pentoxide(P2O5) as raw materials. The synthesized nanoparticles were analyzed by x-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM) methods. The effects of reaction temperature, time and concentration of reactants on the microstructure of the nano-particles have been discussed. The results show that the hydroxyapatite particles synthesized have features of uniform morphology, pure phase and well crystallized nano size. The particle size is in the range of 30-200 nm.