AC4C cast aluminum alloy and zinc-coated steel were friction stir lap welded, and the microstructures and mechanical properties of the joints were examined and analyzed. Experimental results show that the welding speeds have a significant effect on the tensile properties and fracture locations of the joints at a rotational speed of 1500 rpm. When the welding speed is higher than 60 mm/min, the joints fracture in the zinc-coated steel base material and the tensile strength is equal to that of the zinc-coated steel; when the welding speed is lower than 60 mm/min, the joints fracture in the interface and the shear strength is about 50 MPa. The change of the fracture locations is attributed to the presence of large quantity intermetallic compounds adjacent to the interface of the joints. The composition and formation mechanism of the intermetallic compounds and its effect on the mechanical properties of the joints were discussed.