Microstructural development, stress sensitivity and activation energy for superplastic tensile deformation of silicon nitride consisting of rod-shaped grains were investigated. Rod-shaped grains, which oriented randomly before deformation, aligned along tensile direction. Both the stress sensitivity and the activation energy increased during the deformation. The principle deformation mechanism is considered to be the Non-Newtonian viscous flow of grain boundary glassy phase at the former and middle stages. At the last stage (more than 200 % elongation), however, superplastic behavior is considered to be mainly controlled by the solution - precipitation with 2-D nucleation.