Hydroxyapatite (HAp) ceramics have been prepared from alpha-tricalcium phosphate (alpha-TCP) with addition of water or ammonia water by hydrothermal hot-pressing (HHP) method under a condition of 300 degrees C/40 MPa/2 h. When the additive is water, it appears that the HAp grains produced show whisker-like and plate-like features. The bending strength of the HAp ceramic obtained is measured to be 24.5 MPa. When the additive is ammonia water, the obtained HAp ceramic consists of HAp-whiskers only. The bending strength and fracture toughness reach 56 MPa and 1.52 MPa.m(1/2), respectively. Treating alpha-TCP with addition of ammonia water by the HHP method is thus a useful method for in situ fabrication of HAp-whisker ceramics.