Various silicon nitride materials have been prepared by pressureless sintering using MgO/CeO2 as sintering aids. The weight fractions of MgO+CeO2 added were 2, 4, 6, 8, 10, 12, and 14%, respectively, corresponding to MgO/CeO2 weight ratio of 1/1, and constant 10 wt.% of MgO+CeO2 were added in the ratios 0/10, 2/8, 4/6, 5/5, 6/4, 8/2, and 10/0, respectively. The densification effect of the combination of MgO with CeO2 is much better than either MgO or CeO2 alone. The optimum MgO/CeO2 weight ratio is 1/1 and the optimum MgO+CeO2 concentration is 8-10 wt.%, resulting in a relative density of 98.5% and a bending strength of 948 MPa for the pressureless sintered Si3N4-MgO-CeO2 ceramics.