Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) is the main inorganic component of bone and dental tissues. Synthetic HA ceramics are bioactive and biocompatible materials due to the similarity in composition and crystal structure to biomineral HA. The low mechanical strength and fracture toughness of HA ceramics limit their applications to powders, coatings, porous bodies and nonload-bearing implants. It has been found that nanostructured materials have mechanical properties substantially different from their coarse-grained counterparts. We compacted nanosized HA powders under high-pressure and-temperature. The resulting materials were studied using X-ray diffractometry, scanning electron microscopy, and infrared spectroscopy. The microhardness and the fracture toughness of compacted HA ceramics were evaluated using a Vickers indentation method. It is found that all compacted samples under high pressure are composed of nanosized clusters which contain 10-20 nm HA grains. Elongated clusters were observed in the sample densified at higher temperatures. XRD and FTIR results indicate an increase in crystallinity and crystal perfection with increasing temperature. No obvious difference in microhardness was observed between samples, however, the fracture toughness increases with increasing temperature while the translucency of the densified materials is reduced. The difference in mechanical and physical properties of the nanostructured HA ceramics could be explained in terms of grain growth and water evaporation during high-pressure and high-temperature sintering.