In this study, bulk nano-crystalline $Si_3N_4$ ceramics were fabricated by spark plasma sintering (SPS) and their mechanical properties, in particular wear, were investigated. A wide range of grain sizes, from 80 nm and 250 nm were obtained by varying sintering conditions ($1550^{circ}C$-5 min to $1650^{circ}C$-20 min). The elastic modulus of obtained ceramics was ${sim}250$ GPa and hardness was in the range of $13{sim}14$ GPa. The indentation fracture toughness increased from $2.58MPa{cdot}m^{1/2}$ to $3.24MPa{cdot}m^{1/2}$ with increasing sintering temperature possibly due to the elongated grains. Sliding wear tests revealed at least an order magnitude improvement in wear resistance with grain refinement. Microstructure analysis indicated that nano-$Si_3N_4$ specimens worn mainly through delamination and microcracking, while that of coarser specimens revealed severe wear with grain debonding and fracture.