Fine hydroxyapatite (HA) powders were prepared by mechanically activating a mixture of calcium oxide and brushite powders in a high-energy shaker mill. A defective HA phase was formed when the starting powder mixture was activated for less than or equal to 20 h. When the mixture was calcined at 800 degreesC, it was converted to beta -tricalcium phosphate. In contrast, a nanocrystalline HA phase was formed when the mechanical activation was extended to 30 h. The material was transformed to a HA compound (Ca-10(PO4)(6)(OH)(2)) of high crystallinity when it was calcined at 800 degreesC.