Silicon nitride($Si_3N_4)$ is one of the most widely used structural ceramic materials. However silicon nitride is difficult to sinter because of its strong covalent bonding characteristics. In this study, $Si_3N_4$ ceramics were fabricated by spark plasma sintering process with $Y_2O_3$ and $Al_2O_3$ addition to improve the sinterability and the mechanical properties and their phase transformation behavior, microstructure and mechanical properties were evaluated. Fully densified $Si_3N_4$ ceramics could be obtained by spark plasma sintering process at a lower temperature than conventional sintering method. The formation of network microstructure was affected by the addition of $Al_2O_3$ because it could accelerate a to ${alpha}$ to ${eta}$ phase transformation of $Si_3N_4$. As a result, the mechanical properties depended on amounts of $Al_2O_3$ addition. The hardness value increased with increasing ${alpha}$-phase fraction, but fracture toughness value increase with increasing ${eta}$-phase fraction.