Abstract: The dissimilar materials (5A06 aluminum alloy/copper T2) were difficultly welded by conventional fusion welding because of the enormous difference of their thermo-physical performances. The butt joint of dissimilar metals was welded with friction stir welding method. This paper mainly studied on the relative location effect of the base metal on the forming of the FSW joint by analyzing the mechanical properties, and the macro-morphology of weld surface and macrostructures of transverse section of welds. The results indicated that a good Al/Cu FSW joint can be achieved with the proper base metal relative position. When the copper plate was on the retreating side (RS) and the offset of the pin from weld centerline existed, the formation of the welded joints were pretty good, the 'onion' ring structure in the stir zone could be observed clearly and the tensile strength of joint could reach 225 MPa, which was 76.3% of copper T2. Most of the welded joints fractured in the heat affected zone (HAZ) of advancing side (AS) or the nugget zone (NZ).