Deformation mechanism of room-temperature superplasticity in Zn-22wt%Al alloy was investigated by the direct observation during deformation. It was revealed that the dominant deformation mechanism of room-temperature superplasticity was grain boundary sliding. Also, superplastic properties and microstructure of friction stir welded Zn-22wt.%Al alloy were investigated, where Friction Stir Welding (FSW) has received a great deal of attention as a new solid-state welding technique. A sound jointing material was obtained successfully, and extremely fine and equiaxed grains were created in the stir zone. In addition, it was indicated that superplastic forming of the FSWed Zn-22wt.%Al alloy could be feasible. However, the tensile strength and elongation of the joint at room temperature were lower than that of the base material.