An experimental study on superplastic forming behaviors and microstructure characters of commercial magnesium alloy sheet AD I B is presented in this paper. The main experimental results show that the commercial magnesoium alloy AZ31B sheet has superplastic capability. For the received sheet without any pre-processing, the maximum elongation is 295%. The dynamic recrystallization and grain refinement can be found in the case of temperature < 350 degrees C, and the grain growth appears if temperature >= 350 degrees C. The superplastic behaviors can be improved by controlling the dynamic recrystallization and grain refinement. Some experimental results of superplastic free bulging are also presented in this paper. The results show that influence of temperature on forming capability for free bulging is much less than the influences of temperature on tensile elongation. In addition, the maximum principle strain on the top of the bulged dome is less then a half of the tensile elongation.