Hot-pressed nanosized SiC-particle (SiCp)-reinforced Si3N4 composites have been studied with respect to their microstructures, room temperature mechanical properties and thermal shock resistance. The experimental results indicate that the flexural strength, fracture toughness and thermal shock resistance are all increased by the addition of 5 vol% SiCp. Further additions of SiCp, however, have a detrimental effect on these properties. These changes are closely related to the effects of SiCp on the matrix grain morphology. The mechanisms of nucleation and growth of beta-Si3N4 grains in the presence of SiC nano-particles are discussed.