Abstract: T2 copper plates with a thickness of 2 mm were successfully joined by large-load and low-speed friction stir welding. Microstructures and mechanical properties of welding joints were investigated by optical microscopy, scanning/transmission electron microscopy, electron backscatter diffraction, microhardness measurement and tensile testing. The results showed that the thermal cycle was significantly improved in the stir zone, inhibiting the joint softening degree, while heat affected zone was eliminated as well. The stir zone consisted of the ultra-refined grain structures with abundant twin boundaries. As a result, the ultimate tensile strength and elongation were respectively enhanced by 94 % and 69 % compared with those of the base metal. This work provides a simple and effective method to enhance the strength and ductility of friction stir welded copper joints.