The sintering behaviors and resulting mechanical properties of nano-hydroxyapatite (HAP) are studied with a self-developed selective laser sintering (SLS) system. All sintered specimens are fabricated in the sintering range of pure nano-HAP (laser energy densities in 1.6-4.0 J/mm2). The grain characteristics and mechanical properties are tested with scanning electron microscopy (SEM) and Vickers hardness tester respectively after the sintered specimens are polished with diamond paste and etched with 0.5% hydrofluoric acid (HF). The studies reveal improvements in the microstructural and mechanical properties of the sintered specimens with increasing laser energy density. However, abnormal grain growth severely decreases the mechanical properties when the laser energy density passes a certain critical point (3.4 J/mm2). The nano-grain ceramic with the Vickers hardness value of 4.54 GPa is obtained at the critical point. It provides guidance to fabricate nano-grain bone scaffold with pure nano-HAP powder by SLS.