Intrinsic difficulty of metal forming in magnesium alloys must be overcome by new processing. Superplatic injection forming is one of the most promising methodologies. Stroke velocity controlled forming system was developed to evaluate the formability of AZ91 at 573 K both in upsetting and backward-extrusion modes. Uniaxial constitutive equation was used together with consideration of grain growth to predict the stress-strain rate and the stress-stroke relations in upsetting experiments. Double flange, thin-walled cup can be superplastically formed from a cylindrical billet.