Crack formation and oxidation during superplastic deformation of Si3N4 were studied and the superplastic forming ability of Si3N4 was discussed. Tensile deformation tests were conducted under a 1 atm nitrogen atmosphere at 1600 degrees C, and under a constant crosshead speed with an initial strain rate of 2 x 10(-5) s(-1). The microstructures of superplastically deformed specimens were observed by SEM. The relation between chemical composition and microstructure was determined by EPMA. After 280% deformation (at fracture), the formation of regions rich in glassy phase was observed. These regions, supposed to be formed due to oxidation of cracks, appeared just before fracture. The present material is capable of being deformed up to strains of 210%.