Si3N4 ceramics have been fabricated through pressureless sintering and hot-pressing sintering with MgSiN2-Y2O3 or only MgSiN2 as sintering additive, respectively. The effects of MgSiN2 and Y2O3 and sintering methods on sintering properties of Si3N4 ceramics were studied. The results indicate that the bend strength of Si3N4 ceramic with 5.6wt.%MgSiN2-15.8wt.%Y2O3 sintered at 1820°C for 4h could achieve 839MPa. The bend strength of Si3N4 ceramic with 4.76wt.%MgSiN2 produced by hot-pressing sintering at 1750°C for 1h under uniaxial pressure of 20MPa is 1149MPa. The thermal conductivity of the Si3N4 ceramic was 92Wm-1K-1 and could remarkably increase to 129Wm-1K-1 by prolonging the sintering time from 1 h to 12 h. The present work demonstrated that MgSiN2 additives and hot-pressing sintering were effective to improve the thermal conductivity of Si3N4 ceramic.