The c axes oriented hydroxyapatite (HAp) aggregates were synthesized by the hydrolysis of CaHPO4.2H(2)O (DCPD) at 100degreesC in 10 mass% NaOH solution. The HAp was of a plate-like form resembling DCPD. The X-ray diffraction pattern shows that the relative peak intensities of 002 and 004 diffractions increased with the temperature. The crystals in an aggregate were aligned perpendicular to the wide surface of an original plate-like DCPD HAp crystallites grew in the [001] direction perpendicular to the wide surface of plate-like HAp aggregate.