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In this study, aluminum-to-copper welding was performed by friction stir welding (FSW) process and then the mechanical properties of the joints were evaluated and compared with the ones rolled to reductions of 30 and 60 percent. Tensile strengths (UTS) of the joints were 99 MPa, 143 MPa, and 132 MPa, for the initial weld, 30% rolling reduction, and 60% rolling reduction, respectively and in the non-rolled weld specimen, fracture occurred from the aluminum base material but in rolled welds, the fracture occurred precisely from the weld interface. Microstructural studies of the weld region and fracture surface of the specimens showed that the Al₄Cu₉ and Al₃Cu intermetallic compounds, which are the most common intermetallic compounds in this type of dissimilar joining, formed in these areas. The presence of these compounds at the weld interface and propagation cracking during rolling has been one of the important factors in the failure of the weld interface in the rolled specimens. Results of the hardness test also confirmed the existence of these intermetallic compounds. By increasing the percentage of rolling reduction from 30% to 60%, the welding strength decreased due to the increase in the number of micro-cracks of the intermetallic compounds. Finally, it can be said that by choosing the optimal percentage reduction in the rolling process (30%), can be significantly increased (about 43%).  the ultimate tensile strength of dissimilar Al/Cu joints produced by friction stir welding (FSW).