Hydroxylapatite (HA)-nanosize alumina composites were synthesized to study their phase stability and mechanical properties. To make these composites, nanosize alpha-Al2O3 powder was used because of its better sinterability and densification as compared to nanosize gamma-Al2O3. The composites were air sintered without pressure and hot pressed in vacuum at 1100 degrees C and 1200 degrees C. In the composites, HA decomposed to tricalcium phosphate faster after the air sintering than hot pressing. Moreover, hexagonal unit cell volume of HA left in the composites showed that there was more decomposition of HA after the air sintering than hot pressing. It also showed that HA in the composites was OH- and Ca2+ deficient. As the amount of alumina increased, sinterability considerably decreased. Hot pressing at 1200 degrees C resulted in better mechanical properties (mu-hardness and fracture toughness) than the hot pressing at 1100 degrees C. (c) 2006 Elsevier B.V. All rights reserved.