Limited past studies have indicated that Si3N4doped with Sc2O3may exhibit high‐temperature mechanical properties superior to Si3N4systems with various other oxide sintering additives. High‐temperature deformation of this system was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, Sc and O, exist in small amounts at thin grain boundary layers and within secondary phases at triple and multiple grain boundary junctions. The secondary phase is devitrified as crystalline Sc2Si2O7. Deformation of the samples was dominated by cavitation rather than dislocation processes. Thus, the excellent deformation resistance of the samples at high temperature can be attributed to the high refractories and enhanced crystallization of a secondary ph