Abstract: Self-reacting friction stir welding experiments for 4 mm thick 6063-T6 aluminum alloy were carried out in this study. The results indicated that good surface appearance and inner defect-free joints could be obtained. The joint macro morphologies were dumbbell shaped and the joint could be divided into four microstructure zones i.e. base material zone, heat affecting zone, thermal-mechanical affect zone and weld nugget zone. The hardness distribution of all the joints were W-shaped. When the rotation speed was 1 200 r/min and the welding speeds were 400~700 mm/min, the tensile strength of the joint first increased and then decreased. The tensile strength of the optimized joint was 181.64 MPa which equaled 68.5% of that of the base material. All the failed joints fractured in the heat-affected zone at the forward side with a ductile fracture mode.