Dissimilar joining of Al alloys and steel was carried out using diffusion bonding process. The effects of Si and Mg contents of Al alloys and bonding parameters on the interfacial reaction were fundamentally investigated. While the reaction layers consisting of Fe-Al type intermetallic compounds (IMCs) formed at the interfacial region, in the joint with Al alloys including 1mass% Si or more a ternary Fe-Al-Si IMC layer formed at the Al alloy side. The growth of the reaction layers followed the parabolic growth low. A maximum joint strength was obtained at an average reaction layer thickness ranging from 0.5 to 1μm. The thicker reaction layer caused the fracture of the joints at a lower stress because of brittleness of the IMCs, and the thinner reaction layer including un-bonded interface also resulted in a low joint strength. As a result a thin and uniform reaction layer including less un-bonded interface can realize a high joint strength. Since the Fe-Al-Si IMC layer uniformly formed more rapidly than the binary Fe-Al IMCs in the joint with Al alloys including 1mass% Si or more, a higher joint strength was obtained at a thinner average reaction layer. As a result, it was found that the chemical compositions of 6000 series Al alloy controlled to be Mg (0.6 to 1.0mass%) and Si (more than 1.0mass%) were appropriate to obtain the better bonding characteristics.