Abstract: The powdered nano-hydroxyapatite (HA) powders was synthesized by sol-gel method with Ca(NO3)2 · 4H2O and P2O5 as main starting materials and citric acid (CA) as chelating agent. With the influencing mechanism discussed, the influence of CA on the formation process of HA nanopowder and its phase purity and particle size was investigated via DSC, XRD, and SEM. The results showed that CA adding improves the gel's stability and the HA formation process is changed by chelation. The contents of CaO phase in HA nanopowder obtained after CA adding are reduced obviously in comparison with the powder prepared without CA added in precursors. Moreover, CA adding can suppress the agglomeration of powder particles thus making the powder particles of synthesized nano-HA finer and more even with high dispersibility.