Abstract: Five-millimeter thick 6005A-T6 aluminum alloy was butt-welded by friction stir welding with self-designed bobbin tool. The microstructure and properties of the joint were studied. The results show that the weld was formed well. The welded joint consisted of weld nugget, thermo-mechanically affected zone, heat affected zone and base metal. A distinct boundary existed between the heat affected zone and thermo-mechanically affected zone at the advancing side. The microhardness at cross-section of the joint showed a W-shape distribution and reached the lowest value of 60 HV at the interface between thermo-mechanically affected zone and heat affected zone at the advancing side. The tensile strength of the joint was 205 MPa. The joint fractured in a ductile manner at the interface between thermo-mechanically affected zone and heat affected zone at the advancing side. The conditional fatigue limit of the joint was 96.6 MPa. The cracks initiated at the oxide inclusion near the upper surface. The catastrophic rupture region of the fatigue fracture showed dimple morphology.