Silicon nitride ceramics were sintered by pressureless sintering with α-Si3N4 powder as raw material, and Y2O3 combined with MgAl2O4 as sintering aids. The effects of sintering temperature, holding time, content of sintering aids and composition ratio on the densification and mechanical properties of the silicon nitride ceramics were studied. The results show that silicon nitride ceramics can be fully densified by the addition of Y2O3 and MgAl2O4 sintering aids. Using Y2O3 and MgAl2O4 as sintering aids system, when the sintering temperature is 1 600 ℃, the holding time is 4 h, the content of sintering aids is 12.5% (mass fraction), and the mass ratio of Y2O3 and MgAl2O4 is 1∶1, the pressureless sintered silicon nitride ceramics achieve the best comprehensive properties with apparent porosity of 0.21%, the relative density of 98.10%, the bending strength of 598 MPa and Vickers hardness of 15.55 GPa.