Magnesium alloys have been known as the best lightweight metallic materials for various applications of electronic equipments and automobile parts due to high specific strength and stiffness. The needs for wrought magnesium alloys have been increased for the application to structural parts in the form of sheets and bars. However, magnesium has a hexagonal closed-packed (HCP) crystal structure with a limited number of operative slip systems at room temperature, and its formability is restricted to mild deformation. The improvement of the formability of magnesium sheets for real applications is important. In order to increase formability of magnesium sheets at elevated temperature, one promising way is a grain refinement.