Magnesium alloys are attractive for lightweight structural applications in the transportation industry because of their low density and high specific strength and stiffness [1]. With an ultrafine-grained microstructure, they exhibit superplasticity at relatively low temperatures and high strain rates [2]. Friction stir processing (FSP) was used to obtain a microstructure with ultrafine grains in the magnesium alloy AZ31. Microstructures obtained using different rotational speeds are studied. Free bulge forming of the FS processed AZ31 sheets are carried out to evaluate the superplastic behaviour [3]. The model and the evolution equations are, then, implemented into a commercial FE code and different simulations are conducted to correlate the experimental and numerical results for the model validation [4]. The purpose of this study is to investigate the effect of the microstructure on the superplastic behaviour using free bulge forming and FE simulations.