Si₃N₄–O’SiAlON composites using waste-Si-sludge was fabricated by gas-pressure-sintering (GPS) process at 1950°C for 3.5 h. The percent nitridation of waste Si compacts showed a lower value than that of using commercial Si powders caused by the high contents of oxygen in the waste Si particles. Some amounts of Si₂N₂O and Y₂Si₂O₇ phases were also detected in the reaction-bonded Si₃N₄ (RBSN) although main components were α-Si₃N₄, β-Si₃N₄ and residual Si phases. In the post-sintered body by GPS, no residual Si, α-Si₃N₄, Si₂N₂O and Y₂Si₂O₇ phases were detected except the β-Si₃N₄ and O’SiAlON. The grain growth of rod-like Si₃N₄ grains was inhibited by the dispersion of fine O’-SiAlON particles. The fracture toughness and strength of Si₃N₄–O’SiAlON composites were 5.6 MPa·m^1⁄2 and 456 MPa, respectively.