Abstract: In order to investigate the phase transition of silicon nitride (Si₃N₄), α-Si₃N₄ with Y₂O₃ and Al₂O₃ as additives was sintered under 5.2-5.7 GPa at 1 100-1300℃ for 15 rain. The sintered samples were analyzed by X-ray diffraction, scanning electron microscopy and density measurement. The results showed that α→βphase transition started in 1 100-1200℃ and the ratio of transformation from a to β phase increased with increasing temperature and pressure, α-Si₃N₄ was completely transformed to β3-Si₃N₄ under 5.7 GPa at 1300℃. The β-Si₃N₄ was composed of elongated β-Si₃N₄ rod crystals with disordered orientation and had intergranular interlocks and uniform and compact microstructure. Pulled-out crystal grains on the fractured cross sections were obviously observed by SEM. The boundary equations of the two phases were roughly fitted on the base of obtained data. The related mechanism of phase transformation was discussed.