Hydroxyapatite whiskers were synthesized by heating the mixed solution of low concentration of calcium chloride (CaCl2) and dipotassium hydrogenphosphate (K2HPO4⋅3H2O) at 90°C. The whisker aggregates precipitated from the aqueous solution, and the morphology changed from the spherical form to arborescent form and cone-like form with increasing the initial pH of the aqueous solution. During the precipitation of the hydroxyapatite whiskers, the pH of the solution decreased and the solution tended to precipitate the monetite due to the release of H+ ions. To prevent the precipitation of monetite and synthesis the hydroxyapatite whiskers with a good efficiency, it is need to keep the pH of aqueous solution above 4.1 during the precipitation reaction.