An investigation of the pressureless sintering of Si3N4 with MgO-CeO2 revealed that MgO-CeO2 is a much more effective sintering aid for silicon nitride than either MgO or CeO2 alone. The concentration of MgO-CeO2 and the MgO/CeO2 weight ratio have a strong influence on the densification behavior and mechanical properties of the sintered materials. The pressureless sintered silicon nitride with MgO-CeO2 achieved a relative density of 98.5% and a bending strength of 948 MPa.