Si₃N₄ ceramics have become one of the most promising materials for third-generation semiconductor’s ceramic substrates due to its excellent mechanical and thermal properties. In this work, seven rare earth oxides (RE₂O₃, RE=Sc, Lu, Yb, Y, Gd, Nd, La) and non-oxides (MgSiN₂) were used as composite sintering additives to prepare Si₃N₄ ceramics with high strength and high thermal conductivity by hot pressing sintering and annealing. The effects of RE₂O₃ types in the composite additives on the phase composition, microstructure, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were systematically studied. The mechanical properties of Si₃N₄ ceramics after hot pressing sintering were superior to that without hot pressing, and the bending strength of the sample with Nd₂O₃-MgSiN₂ additive reached (1115±49) MPa. After annealing, the thermal conductivity of Si₃N₄ ceramics increased significantly, and gradually increased with the decrease of the rare earth ion’s radius. The thermal conductivity of the sample with Sc₂O₃-MgSiN₂ additive increased from 54.7 W·m^-1·K^-1 to 80.7 W·m^-1·K^-1 (increased by 47.6%) after annealing. The results indicate that Sc₂O₃-MgSiN₂ is expected to be a new composite additive for the preparation of high strength and high thermal conductivity Si₃N₄ ceramics as compared to the internationally accepted combination of Y₂O₃-MgSiN₂ and Yb₂O₃-MgSiN₂ additives.