Nanograined Si3N4 ceramics with Y2O3-Al2O3-MgO as sintering additive exhibited superplastic elongation of >300% at 1923K with an initial strain rate of 5x10(-4)s(-1). Flow stress was less than 4MPa up to elongation of 130%. In the later stage of deformation, the flow stress increased with strain due to the grain growth and the alignment of elongated grains. The volume fraction of glassy phase significantly reduced due to vaporization of glassy phase. The chemical analysis by TEM/EDS revealed that the chemical composition of the near Si3N4 grain surface was different from that of the interior of the glass pocket at multigrain junctions due to absorption of additive cations on the Si3N4 grain. The chemistry of intergranular glassy phase changed significantly in the later stage of deformation accompanying the microstructural evolution. It is suggested that the flow stress, which depends on the viscosity of intergranular glassy phase, varies with the change in glass chemistry during deformation.