Grain refinement of magnesium alloy by Friction Stir Processing (FSP) was investigated. It is assumed that dynamic recrystallization (DRX) is occurred by frictional heat and plastic flow during FSP. This process is the effective method of the grain refinement for light metals. In this study, FSP was conducted to cast Mg alloys for and the difference of the grain refinement by DRX in these alloys was examined. As a result, in comparison with commercial Mg-Al-Zn alloy and Mg-Y-Zn alloy have finer microstructure. The grain size of FSP-ed Mg-Y-Zn alloy was ~1.7 [/m], however, that of AZ31 alloy was 20~30 [/m].