The noble synthesis method for hydroxyapatite (HAp) nanoparticles was exploited using a fairly simple reaction of Ca(OH)(2) and H(3)PO(4), which does not generate residual harmful anions and consequently does not need an additional washing process. HAp nanoparticles were found to yield from dicalcium phosphate dehydrate (DCPD) as the only intermediate phase, which was monitored by in situ observation study using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), (1)H and (31)P magic-angle spinning (MAS) NMR. Furthermore, we found that the phase evolution of HAp was preceded by heteronucleation of HAp onto the DCPD surface. The combination of scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-ES) analysis gave more information on the HAp crystallization process, which was found to be retarded by the residual Ca(OH)(2) and slow diffusion process of Ca ions into the interface between HAp and DCPD. These results demonstrate that the synthesis of pure HAp nanoparticles with high throughput can be achieved by controlling the residual Ca(OH)(2) and diffusion process of Ca ions.