Calcium deficient hydroxyapatite (Ca-def HA) whiskers with various Ca/P ratios were synthesized by a homogeneous hydrothermal precipitation method using acetamide. They had high crystallinity for Ca/P > 1.54, with a mean length of 79-113 mu m and an aspect ratio of 85-103. No whisker entanglement and aggregation were found in the products. Thermal analyses revealed that the Ca/P ratio and crystallinity of the products showed significant effects on the crystalline phase, constitution and morphological stability of the whiskers at an elevated temperature. The whiskers with Ca/P > 1.54 were morphologically stable at temperatures below 1200 degrees C, with minor TCP appearance after heat-treatment at 800 degrees C; no further decomposition of the whiskers was found with increasing temperature. Such whiskers are possibly one promising reinforcing materials for preparing highly reliable apatite ceramics and composites either in dense or porous block materials with high mechanical properties and good enhanced biological performance. (C) 2011 Elsevier B.V. All rights reserved.