|Mesoporous hydroxyapatite (HAp) powders were synthesized by a hydrothermal method. An aqueous solution (0.5 M) of diammonium hydrogen phosphate [(NH4)2HPO4, DAHP] at pH 11-12 was mixed at DAHP: NH2(dendrimer) = 1:3 by mole with an aqueous solution of amine-terminated 4th generation poly(amido amine) (PAMAM) dendrimer as a porogen. After allowed to stir for 30 min, the mixed solution was added dropwisely to an aqueous solution of calcium nitrate [Ca(NO3)2.4H2O] at pH 11-12 with vigorous stirring at room temperature. Then the calcium nitrate:DAHP mole ratio was adjusted around 1.67. The resultant white gelatinous precipitates were then incubated for 24 h at 150 oC in an autoclave. The precipitates were separated by filtration, washed with water and ethanol in sequence and dried. The existence of nanopores was confirmed with a transmission electron microscope and a low angle X-ray diffraction spectroscopy.