Nanoparticulate systems have been studied for targeted and controlled release of therapeutic agents; and size is one of the major determinants of their in vivo clearance kinetics by the MPS macrophages. As such, it is important to control the size of hydroxyapatite nanoparticles during synthesis. The results show that the size of hydroxyapatite nanoparticles, synthesized through chemical precipitation, increases with increasing synthesis time. Particle sizes were also observed to increase in a linear correlation with temperature. Crystallinity and carbonate-substitution of the nanoparticles also increased with temperature. Hydrothermal, performed as a post-synthesis treatment, improves particle morphology, giving particles with regular surface contours, well-defined sizes and lower particle agglomeration. By controlling synthesis temperature and time, hydroxyapatite nanoparticles with well-defined sizes and morphology can be obtained.