4 mm thick plates of 5A06 aluminum alloy and 2 mm thick plates of 316L stainless steel are successfully lap-butt jointed using friction stir welding (FSW). In order to solve the lack of penetration and hook problem, a lap-butt configuration is adopted based on the principle of ″height difference-pin offset″. The interface which exhibits characteristics of smoothness, no hook and incomplete penetration is found by observing the microstructure of the joint. There are steel particle reinforced aluminum matrix composite tissue structure and river pattern structures near the interface of the welds. Meanwhile, the element diffusion and the generation of intermetallic compound occur in the interfacial zone, and the thickness of the layer is about 3 μm. The microhardness value in the bonding interface reaches the maximum. The tensile strength of the joint reaches 89.7% of the aluminum alloy base metal strength.