Abstract: Dissimilar materials, aluminum alloy and mild steel, were joined by friction stir welding, and the weld appearance and mechanical properties of joint were analyzed.The results shown that a deformation-free joint with good weld surface appearance can be obtained with suitable welding conditions for butt-joint and lapjoint of the dissimilar materials.For the butt joint, the mild steel and aluminum alloy were mixed well within the weld cross section, and the two kind of materials was intercalated on the middle plane parallel to the weld surface.The microhardness is higher than the mild steel at local place within weld nugget, which perhaps is the formation of intermetallic compound at these points. The joint is fractured from the edge of the nugget at the steel side for the buttjoint. For the lap-joint, the cross section appearance at the weld is somewhat like pincers or hook, that is, the steel is riveting into the aluminum alloy with a curved shape.The two kind of materials could reach plastic joining and the joint have a rather good shear properties.