Abstract: Effect of different travel speeds on the root-defects and mechanical properties of friction stir welded 5.1 mm thick A6082-T6 alloy joint was investigated in this study. The root-defect and microstructure of joints were observed by optical microscope, the tensile and bending tests of joints with different travel speeds were conducted. The results showed that the tendency of incomplete penetration defect and the lowest hardness of softening area in joints were increased with increasing travel speed. The tensile strength of FSW welded A6082-T6 alloys was increased with increasing travel speed from 50 mm/min to 400 mm/min. When travel speed was further increased to 600 mm/min, the tensile strength of joint decreased because of serious incomplete penetration defect. It was found that the small size root-defect had little effect on the tensile property whereas a deleterious effect on bending properties of the friction stir welded A6082-T6 alloy joint.