The microstructures and mechanical properties of laser penetration welded joints of overlap steel-on-aluminum were investigated. The structure of the intermetallic compound layer without interlayer consists of FeAl and FeAl₃ phases. After the Ni-foil was added, the thickness of the intermetallic compound layer and the content of the brittle and hard Al-Fe phase decreased significantly, and some new phases of Al_0.9Ni_1.1 and FeNi were formed. It was found that the Ni interlayer enhanced the tensile property of the joint by about 40% and decreased the microhardness of the intermetallic compounds, which is attributed to the improvement of the toughness of the welded joint made by the Ni interlayer. It is an effective way to improve the mechanical properties of the laser welding joint by adding a nickel interlayer to improve the metallurgical reaction.