In this paper,6061-T6 aluminum alloy and pure copper are welded by friction stir welding and the effect of rotation speed on the microstructure and mechanical properties of dissimilar joints is studied. When the welding speed is 30 mm/min and the rotation speed is in the range of 1 200 r/min to1 800 r/min,sound joints without defects can be obtained. The complex structures are formed in the nugget zone where a large number of broken Cu particles are stirred into. Al,2,Cu,Al,4,Cu,9, and AlCu intermetallic compounds are found in the nugget zone.The intermetallic compound layer is formed by mutual diffusion between aluminum and copper at the interface. With the increase of rotation speed,the compounds layer gradually becomes thicker. Due to the grain refinement,solid solution strengthening and the appearance of the IMCs,the average microhardness of the nugget zone is higher than that of aluminum and copper sides,and the microhardness peak appears in the nugget zone.Meanwhile,the tensile strength of the joint firstly increases and then decreases. The tensile strength of the optimal joint is 183 MPa and the joint efficiency is up to 71.8% of Cu BM. The fracture occurs in the heat affected zone of Al side,and the fracture mode is a ductile fracture.