Abstract: The welding tools with shoulder coupled with different number (0, 2, 4, 6) of asymptote grooves were used in friction stir welding aluminum alloys. By using insert tracer technique, the effect of shoulder profile on the plastic flow of weld was studied. The results showed that, with increasing the number of asymptote groove, the size of nugget zone and the height of transferred tracer material on two sides increased firstly and decreased then. Also, the height of material transfer on advancing side was larger than that on retreating side in a same weld. When the tool with a shoulder of 4 asymptote grooves was used, the loose defect disappeared. Meanwhile, the area and width of nugget zone and transferred material height on two sides were the largest. Increasing properly the number of asymptote groove can increase the plastic deformation and the volume of transferred plastic metal, which finally improves the weld quality.