The crystal growth and morphology of the nano-sized hydroxyapatite (HA) powders synthesized from dicalcium phosphate dihydrate (CaHPO(4)(.)2H(2)O, DCPD) and CaCO3 have been investigated. The nano-sized HA powders were obtained using the hydrolysis of DCPD and CaCO3 with 2.5 M NaOH(aq) at 75degreesC for 1 h. The only product synthesized from DCPD is HA, and the crystallinity of the HA is improved with increasing annealing temperature. The XRD results show that when heated at 600degreesC for 4 h, the mixture of the HA and CaO is obtained with CaCO3 addition, having the Ca/P ratio of 1.67. However, when the mixture is heated at 800degreesC for 4h, besides the HA and CaO, the NaCaPO4 phase also shows up. (C) 2004 Elsevier B.V. All rights reserved.