Friction stir process is one of the severe plastic deformation techniques which causes microstructure refinement and hence change in mechanical properties of the specimen. Temperature, strain and strain rate which are the results of this process, causing dynamic recrystallization and followed by shift in mechanical properties. Magnesium is the lightest structural alloy which is used in a wide range, in the automobile and aerospace industries. In this research, the overlapping friction stir process is applied on the surface of AZ31 alloy. The overlapping is conducted in 50 and 0 percentages. In order to reduce the temperature which is generated in the process, rapid cooling was used. After friction stir processing in the 0 percent overlapping the grains refine from 18 micrometer in the base metal to 7 micrometer with a uniformer structure than the base metal, ultimate tensile strength and elongation increased 29.9% and 19.8%, respectively with respect to the base metal. The structural uniformity of 50 percent overlapping was less than that of 0 percent overlapping, ultimate tensile strength and elongation increased 19.98% and 3.9%, respectively with respect to the base metal.