Si3N4 ceramics were prepared via gas pressure sintering under nitrogen pressure of 1.0 MPa at 1 850 ℃ for 3 h with Y2O3 and MgSiN2 as sintering aids. The effects of MgSiN2/Y2O3 mole ratio and their total amount on the porosity, microstructure, mechanical properties and thermal conductivity of Si3N4 ceramics were investigated. The results show that the growth of rod-like β-Si3N4 grains was more sufficient and the growth of abnormal grain decreased with the increase of Y2O3 content at the constant total content of MgSiN2 and Y2O3. Si3N4 ceramics with the optimal properties were prepared at a molar ratio of MgSiN2:Y2O3 of 5:2 and the total amount of sintering aids of 10.5%, having the bending strength and thermal conductivity of 510 MPa and 67.01 W?m--1?K-1, respectively.