The microstructures and mechanical properties of $Si_3N_4$ ceramics produced by nitridation and post-sintering using semiconductor-waste-Si sludge were investigated. Lots of microcracks were observed in the waste-Si powders which contained some amounts of amorphous $SiO_2$. The nitridation rate of waste-Si compacts showed lower value than that of commercial Si powder compacts. The nitridation rate was increased with increasing nitridation temperature and then the percent of nitridation at 1470$^{circ}C$ showed 98%. The phases of $Si_3N_4$ in the reaction-bonded bodies were mixed with ${alpha}$ and ${eta}$-type, and small amounts of $Si_2N_2O$ phase while those after post-sintering were ${eta}$-$Si_3N_4$ and ${alpha}$-Sialon. The sample post-sintered at 1950$^{circ}C$ showed the fracture toughness of 5.6 $^MPa{cdot}m^{1/2}$ and the fracture strength of 497 MPa which were lower than those of sintered body using commercial Si powder possibly due to the formation of ${alpha}$-Sialon phase.