Fine-grained AZ31 magnesium alloy sheets were prepared through hot rolling process. The superplastic properties of hot-rolled AZ31 Mg alloy was examined by uniaxial tensile tests at a temperature range 250 similar to 450 degrees C and strain rate range 0.7x10(-3)similar to 1.4-10(-1)s(-1). Optical and scanning electronic microscope (SEM) were used to observe the microstructure evolution and fracture behavior in superplastic deformation of AZ31 Mg alloy and the values of deformation activation energy at various temperatures were calculated. It is demonstrated that, the hot-rolled AZ31 alloy begins to exhibit superplasticity from 300 degrees C and a maximum elongation of 362.5% is obtained at 400 degrees C and 0.7x10(-3) s(-1). In the temperature range 300 similar to 400 degrees C, the dominant superplastic deformation mechanism is grain boundary sliding (GBS) controlled by grain boundary diffusion and the influence of temperature on the fracture behavior of AZ31 Mg alloy is characterized by the change from dimple-aggregating type to intercrystalline one.