Abstract: Single-side and double-sided welding tests of T2 copper with thicknesses of 30 mm and 50 mm were done by friction stir welding, while the microstructure and mechanical properties of welds were analyzed. The results show that butt joints with good superficial shaping quality, almost no deformation, and no internal defects can be achieved within certain ranges of process parameters. The average tensile strength of the T2 copper single-side weld with a thickness of 30 mm is 177.2 MPa, reaching 81.7% of the strength of the base metal, while the average elongation after fracture is 25.4%. The microhardness distribution of the weld transverse section fluctuates obviously, with the minimum locating at the bottom of the heat affecting zone along the advancing side, where it is welding weakness.