Abstract: In this work, maintaining the welding speed at 95 mm/min, effect of rotation speed on formation quality and tensile properties of friction stir welding dissimilar of Mg/Cu alloys joints was studied. The results show that, there are some flash on the weld surface and obvious tunnel channel defect on the bottom of the weld nugget zone when a low rotation speed of 750 r/min is applied. With increasing rotation speed to 950 r/min, the weld becomes smoother, the size of the weld mixed zone enlarges and the tunnel channel defect in the weld mixed zone disappears. The micro-structure of the weld mixed zone is comprised of broken copper-magnesium alloys and a small amount of Mg₂Cu intermetallic compound. Moreover, the tensile performance of the joint is the best with the average tensile strength high up to 81.7 MPa. However, continuously increasing the rotation speed to 1180 r/min, some small holes form on the bottom of the weld mixed zone, which is not conducive to the joint formation.