Properties of hydroxyapatite (HA, Ca10(PO4)6(OH)2), including bioactivity, biocompatibility, solubility and adsorption could be tailored over wide ranges by the control of particle composition, particle size and morphology. In order to satisfy various applications, well-crystallized pure HA nanoparticles were synthesized at moderate temperatures by hydrothermal synthesis, and HA nanoparticles with different lengths were obtained by adding organic additives. X-ray diffractometry (XRD) and Fourier transform infrared (FTIR) spectrometry were used to characterize these nanoparticles, and the morphologies of the HA particles were observed by transmission electron microscopy (TEM). The results demonstrate that shorter rod-like HA particles can be prepared by adding cetyltrimethylammonium bromide (CTAB), as the additive of CTAB can block the HA crystal growth along with c-axis. And whisker HA particles are obtained by adding ethylenediamine tetraacetic acid (EDTA), since EDTA may have effect on the dissolution-reprecipitation process of HA.