Preparation of colloidal hydroxyapatite (HAp) particles under body fluid conditions was investigated with focusing on the effect of preparative conditions on crystallinity of the resulting particles. Tris(hydroxymethyl)aminomethane was added to 1.5SBF (a solution having 1.5 times higher ion concentrations than those of a simulated body fluid, SBF) to increase the solution pH, which resulted in induction of homogeneous nucleation of HAp in the solution. Colloidal HAp particles having diameters about 300. nm were obtained. When the reaction was proceeded at 70 °C and the sample was dried by heating, it was effective to obtain HAp particles having high crystallinity. Experimental results support that remaining water in the sample contributed to increase HAp crystallinity.