Transparent hydroxyapatite (HAp) ceramics with the grain size ranging 86-1 300 nm were successfully synthesized by spark plasma sintering (SPS) at 925-1 200 ℃.All the sample achieved final density higher than 99.7％.The phase stability was identified by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).The experimental results indicate that there is no decomposition or dehydroxylation during the SPS processes.The influences of microstructure refinement on the hardness were investigated using HallPetch (H-P) relationship,and the hardness of transparent HAp ceramic increases with the decrease of grain size.It is demonstrated that the grain boundaries and defects play important roles on the hardness.