A powder mixture of ultrafine beta -SiC-35 wt% alpha -Si3N4 containing 6 wt% Al2O3 and 4 wt% Y2O3 as sintering additives were liquid-phase sintered at 1800 degreesC for 30 min by hot-pressing. The hot-pressed composites were subsequently annealed at 1920 degreesC under nitrogen-gas-pressure to enhance grain growth. The average grain-size of the sintered bodies were ranged from 96 to 251 nm for SIC and from 202 to 407 nm for Si3N4 which were much finer than those of ordinary sintered SiC-Si3N4 composites. Both strength and fracture toughness of fine-grained SiC-Si3N4 composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine-grained region (less than or equal to 250 nm for SIC and less than or equal to 400 nm for Si3N4) was beneficial for mechanical properties of the composites. The room-temperature flexural strength and fracture toughness of the 8-h annealed composites were 698 MPa and 4.7 MPa . m(1/2), respectively.