Calcium phosphate powders with nano-sized crystallinity were synthesized by neutralization using calcium hydroxide and orthophosphoric acid with it lie assistance of citric acid. The influence of processing parameters, Such as free or additive citric acid, synthetic temperature and ripening time, on the crystallinity of hydroxyapatite were investigated. The results of X-ray diffraction and microstructure observations showed that the crystallinity and morphology of nano-sized hydroxyapatite particles were influenced by the presence or absence of citric acid. It Was found that the crystallinities and crystallite sizes of hydroxyapatite powders prepared with the additive citric acid increased with increasing synthetic temperature and ripening time. Especially, the crystallinities of(h k 0) planes were raised and more homogeneously grown particles were obtained with increasing synthetic temperature. (C) 2008 Elsevier Ltd. All rights reserved.