Powder mixtures of beta -SiC-alpha -Si3N4 in a weight ratio of 1 : 1 containing 5-20 wt% Y-Mg-Si-Al-O-N oxynitride as a sintering additive were liquid-phase sintered at 1800 degreesC for 3 h by hot-pressing. These materials had a microstructure of "in situ-toughened composites" as a result of the alpha --> beta phase transformation of Si3N4 during sintering. The introduction of larger amount of additives accelerated the grain growth of elongated beta -Si3N4 grains with higher aspect ratio, resulting in the improved fracture toughness and strength. Typical flexural strength and fracture toughness of SiC-Si3N4 composites containing 15 wt% oxynitride glass were 860 MPa and 5.7 MPa . m(1/2), respectively.