"Compound casting" was used for production of lightweight Al/Mg couples. In order to prepare the Al/Mg couples using this process, each of the aluminum and magnesium molten metal was cast around solid cylindrical inserts of the other metal. After solidification, the interfacial microstructure and shear strength of the joint were studied. Characterization of Al/Mg interface by an optical microscope and scanning electron microscope showed that in the case of casting aluminum melt around a magnesium insert, a gap is formed at the interface, while in the process of casting magnesium melt around an aluminum insert, a relatively uniform interface composed of three different layers is formed at the interface. The results of the X-ray diffraction, energy dispersive X-ray spectroscopy, wavelength dispersive X-ray spectroscopy, and microhardness analysis of the interface showed that these three layers are mainly composed of high-hardness Al-Mg intermetallic compounds. Furthermore, it was found that the thickness of the interface is not constant throughout Al/Mg joint, and varies gradually from 190 mu m at the bottom to 140 mu m in the middle and 50 mu m at the top of the sample. The results of shear strength tests obviously showed that the strength of the interface depends on the interface thickness and increases by decreasing the thickness of the interface.