The superplasticity of magnesium alloy is important in industrial application. However the superplastic deformation of casting magnesium alloy is hard to be realized. In this paper, the stress–strain behaviors of casting AZ31 magnesium alloy with various strain rates at different deformation temperatures were investigated. The alloy was tested in the tensile condition with initial grain size of 25μm. It was found that the elongation of the alloy at 400°C with ε& = 4.25×10-4 s-1 is almost 200%. According to the results of uniaxial tensile experiment, the alloy exhibited superplastic deformation behavior with the slow stain rate in a temperature range of 350 to 450°C. The microstructures deformed and undeformed samples were observed with aid of optical microscope.