Abstract: Bobbin tool friction stir welding was applied to join 3 mm thick 6063-T4 aluminum alloy. When the tool rotation speed was 600 rpm and the welding speed was in range of 100 to 300 mm/min, the sound weld with good formation and without defects can be obtained. In the stir zone of the joint, the hardness is distributed uniformly. In the thermal-mechanically affected zone and thermal affected zone, the hardness at the upper and lower layers was higher than that at the middle layer. The location which was in the thermal-mechanically affected zone and near the stir zone, and the boundary between the thermal-mechanically affected zone and the thermal affected zone were the two weak areas of the joint. With the increase of welding speed, the hardness and the tensile strength of the joint firstly increased and then decreased. The tensile strength of the optimal joint was 174 MPa and the joint efficiency was up to 87%. The fracture location was in the heat affected zone.