Ca(OH)(2) and H3PO4 were used to synthesize nano-sized hydroxyapatite. In the process of acid-base reaction, citric acid or sodium citrate was added to influence the crystallinity of hydroxyapatite. Formation of Ca-citrate complex resulted in variations of ionic concentrations and ionic activity products for solutions with different citrate additions. The degree of supersaturate was found higher for solutions with less citric acid or more sodium citrate addition. Consequently, hydroxyapatite particles with smaller size were gained. These crystalline behaviors were investigated and explained by mechanism of crystal nucleation and growth in solutions. (c) 2008 Elsevier B.V. All rights reserved.