The densification and phase transformation of high α-phase silicon nitride powder synthesized by SHS with a combination of Y2O3 and Al2O3 sintering additives were investigated. The densification process occurred rapidly from 1400 to 1500 °C and was nearly finished at 1500 °C. However, the α-β transformation of silicon nitride progressed rapidly from 1500 to 1600 °C, and completed at 1600 °C. The phase transformation of silicon nitride lagged behind the densification of Si3N4 ceramics. It showed that the solution and precipitation process did not play the main role in the densification of silicon nitride. The sufficient amount of liquid phase was crucial to complete the densification.