Using friction stir processing to produce fine-grained microstructure in AZ31 magnesium alloy. The superplasticity of base metal and fine-grained AZ31 magnesium alloy with different grain sizes were compared. The results show that microstructure of the AZ31 hot-rolled plate with an average grain size of 7.67μm is refined to 0.94μm~3.21μm. The maximum elongation of the BM is 630% at 450℃ and 5×10-4/s-1, The maximum elongation of the FSP AZ31 alloy is 405% in the same situation. Grain size have no linear relationship with superplastic performance. Superplastic deformation mechanism is mainly based on grain boundary sliding. Twin crystal also affects the deformation. The failure mechanism is cavity generation, growth and coalescence.