Abstract: The welding parameters were optimized based on the response surface method in order to improve the mechanical properties of the zero-weld-thinning friction stir welding joints. The response surface model and regression analysis were built. The results showed that zero-weld-thinning friction stir welding was a well-formed welding process. The tensile properties of the joints were obviously influenced by the welding parameters, the rotation speed and welding speed had more significant effect. High rotation speed and moderate welding speed should be selected to get better welding joint without any defects. Under the condition of the rotation speed of 1 000 r/min, the welding speed of 200 mm/min and the shoulder plunge depth of 0.25 mm, the tensile strength of the joints reached to the maximum 363 MPa, which was 94.3 percent of the base metal. The elongation was 11.2%. In comparison to conventional friction stir welding, the mechanical properties of the zero-weld-thinning friction stir welding joint were more homogeneous along thickness direction.