Abstract: A series of controlled friction stir welds had been success-fully made using 2.5 mm thick AZ31 magnesium alloy.The mechanical properly of friction stir welds is largely depends on the welding parameters and welding process.The relationship between the rotation speed/the traverse speed and the ultimate strength of as-welded joint was studied using tensile and bend testing.The results show that sound joints had been made at wehling speed 75~150 mm/min and rotation speed 900~1500 r/min.The paper presented a new evaluating parameter,the ratio of the rotation speed and the welding speed (R/v)——the rotation amount per unit length,and combined this new parameter with the joint quality in place of the total effect caused by the rotation speed and the welding speed.Results indicated that sound joint can be made when R/v=8~15,and the new evaluating parameter can be regarded as reliable when the value of v and R are in good range.