Polycrystalline hydroxyapatite (HAP) ceramics were densified by hot pressing. The effects of thermal treatments and of a sintering additive (Na3PO4) on the microstructure, flexural strength and fracture toughness were investigated. Hot pressing without additive resulted in dense HAP having a Small average grain size (below 0.5 pm). Spontaneous microcracking of the material was also noted. This originated from the thermal expansion anisotropy of HAP crystals. The presence of the sintering aid promotes grain growth. Dense materials exhibited mechanical properties depending on the microstructure. The highest values obtained were 137 M Pa and 1.2 M Pa~/~ for the flexural strength and fracture toughness, respectively. A decrease of both strength and toughness was observed with increasing average grain size. This behaviour is attributed to the weakening of the grain boundaries by either the development of initial microcracking or the Na3P04 addition. It is concluded that hot pressing is very useful to elaborate dense HAP having good mechanical characteristics.