Silicon nitride with high thermal conductivity was obtained by two-step sintering method using YbH₂-MgO as sintering additive. The effect of YbH₂-MgO on shrinkage behavior, phase composition, microstructure, thermal conductivity, and flexural strength was investigated. The nitrogen-riched and oxygen-lacked oxynitride liquid phase was generated owing to the elimination of SiO₂ by YbH₂. Both the removal of lattice oxygen and the growth of β-Si₃N₄ were stimulated by the liquid phase. Therefore, compared to Yb₂O₃-MgO doped sample, silicon nitride with enlarged grains, purified lattice and reduced intergranular phase was obtained. Ultimately, its thermal conductivity increased by 13.7% from 115.32 to 131.15 W·m^-1·K^-1 after sintering at 1900 ℃ for 24 h by substituting Yb₂O₃ with YbH₂. Although the replacement of Yb₂O₃ by YbH₂ leads to enhanced flexural strength at low temperature, it tends to reduce the flexural strength at high temperature. The optimal flexural strength of (1008±35) MPa was achieved after sintering at 1800 ℃ for 4 h. This variation related mainly to the exaggerated bimodal microstructure. This work signifies that YbH₂-MgO is effective for obtaining Si₃N₄ ceramics with both high flexural strength and high thermal conductivity.