Abstract: The joining of dissimilar metals, T2 copper and Q235 mild steel plates with 10 mm thickness, is carried out in friction stir welding.Excellent welds can be gained when copper is fixed at the retreating side, but defects can be produced in welds when copper is fixed at the advancing side.The pin shapes influence the flow of the plasticized metal in the weld, which results in the variety of the morphology of the weld.If the screw thread on the pin is clockwise, the metal around the pin will move upwards to the root of the pin, which causes that the shoulder affected zone is clear and the weld defects would form at the lower part of the weld section.If the screw thread on the pin is counter-clockwise, the metal around the pin will move downwards, which drives the metal around the pin tip to move around and upwards.The onion ring pattern, which appears like lamellar structure, is observed in the stir zone.The shoulder-affected zone is not clear;the weld defects will form at the upper part of the weld section.Various pin offsets will affect the flow of weld metal.If an optimization of the process parameters is performed, defect-free joints can be formed.The tensile test results show that the maximum joint tensile strength can reach 233 MPa, which is 95% of the parent materials of copper, and the fracture happens in the HAZ of copper.The maximum tensile strength of the nugget zone can reach 296 MPa, which is very considerably larger than that of the parent materials of copper.