Silicon nitride sintered bodies composed of finegrained α-Si3N4 and β′-sialon were fabricated by the gas-pressing sintering method. The silicon nitride sintered body showed a very high strength of about 2.0 GPa at room temperature, shock resistance of about 20kJ·m-2 (Charpy impact value) and 3-point bending fatigue limit of about 1.3 GPa.