The purpose of the present work is to toughen the grain boundary phase of Si3N4 ceramics by forming a nano composite grain boundary phase of SiC particles. The nano SiC particles form through a reaction between the coated carbon and the surface oxidation phase in Si3N4. The carbon is coated through thermal decomposition of CH4 gas. The sintcred bodies arc obtained by hot-pressing at 1800 °C for one h. By increasing the carbon content, both fracture toughness and bending strength initially increase, then reach the maximum value at the same amount of carbon content, and decrease. The carbon content at which the fracture toughness and bending strength exhibit the maximum value increases with the increase in the surface oxygen content. The precipitated SiC particles dispersed in a grain boundary phase, at a Si3N4-Si3N4 interface or in a Si3N4 grain. The locations and size (less than 30 nm) of the SiC particles are examined by TEM and HREM observations. As the grain boundary is toughened by the precipitated nano SiC particles in the grain boundary phase or at the Si3N4-Si3N4 interface to help Si3N4 grain bridging without any glassy phase, these SiC particles block or resist the intergranular fracture.