Si3N4 ceramic was densified at 1900 degrees C for 12hours under 1MPa nitrogen pressure, using MgO and self-synthesized Y2Si4N6C as sintering aids. The microstructures and thermal conductivity of as-sintered bulk were systematically investigated, in comparison to the counterpart doped with Y2O3-MgO additives. Y2Si4N6C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO2, resulting in enlarged grains, reduced lattice oxygen content, increased Si3N4-Si3N4 contiguity and more crystallized intergranular phase in the densified Si3N4 specimen. Consequently, the substitution of Y2O3 by Y2Si4N6C led to a great increase in similar to 30.4% in thermal conductivity from 92 to 120Wm(-1)K(-1) for Si3N4 ceramic.