This paper discusses the dependence of the mechanical properties and microstructure of sintered hydroxyapatite (HA) on the sintering temperature and pressure. A set of specimens was prepared from as-received HA powder and sintered by using a spark plasma sintering (SPS) process. The sintering pressures were set at , , and , and sintering was performed in the temperature range from at each pressure. Mechanisms underlying the interrelated temperature-mechanical and pressure-mechanical properties of dense HA were investigated. The effects of temperature and pressure on the flexural strength, Young’s modulus, fracture toughness, relative density, activation energy, phase stability, and microstructure were assessed. The relative density and grain size increased with an increase in the temperature. The flexural strength and Young’s modulus increased with an increase in the temperature, giving maximum values of and , respectively, at a critical temperature of and , and the fracture toughness was at at . Increasing the sintering pressure led to acceleration of the densification of HA.