The pressureless sintering was investigated to fabricate dense Si3N4/SiC nanocomposites from commercially available Si nitrided Si3N4 powder. MgAl2O4+ZrO2 (totally 10 and 15wt%) or Y2O3+Al2O3 (totally 10.5 and 14wt%) and 5-30vol% SiC were used as sintering additives and second phase dispersions, respectively. With increasing SiC content, the density of the composite containing less amount of sintering additives decreased linearly, while the large amount of sintering additives decreased gradually the density or kept it constant. The composites with higher SiC content presented a relatively smaller Si3N4 grain size, because the Si3N4 grain growth was inhibited by SiC particles that pinned the grain boundary movement. While decrease in strength was observed for the composites with less amount of sintering additives, an improvement of the strength was found for the composites with high amount of sintering additives. The strength change of the composites was consistent with their densification behavior and microstructural observation. Composite with 20vol% SiC exhibited high strength up to 1GPa. With increasing SiC content, hardness increased but toughness decreased. It is concluded that the results of this study provide an economic way to fabricate the Si3N4/SiC nanocomposite with high performance.