Uniaxial tensile tests were carried out in the temperature range of 250-450 ℃ and the strain rate range of 0.7×10-3-1.4×10-1s-1 to evaluate the superplasticity of AZ31 Mg alloy. The threshold stress which characterizes the difficulty for grain boundary sliding was calculated at various temperatures. The surface relieves of superplastically deformed specimens were observed by using a scanning electronic microscope (SEM). Results show that, at the temperature of 400 ℃ and strain rate of 0.7×10-3 s-1, the strain rate sensitivity exponent, i e, m value reaches 0.47 and the maximum elongation of 362.5% is achieved. Grain boundary sliding (GBS) is the primary deformation mechanism and characterized by a pronounced improvement in the homogeneity with increasing temperatures. A large number of filaments were formed at the end of deformation and intergranular cavities were produced with the necking and fracture of filaments. Finally, the model for the formation of intergranular cavities was proposed.