The effects of alloying elements on the interfacial microstructure of the dissimilar Al/steel joints were discussed using SEM, EPMA, TEM and tensile test. For Si (and Cu)-added 6000 system aluminum alloy, EPMA analysis showed that Si (and Cu) was enriched in the reaction layer, which were formed during diffusion bonding. SAED (Selected area electron diffraction) pattern identified the reaction layer at the interface as Al-Fe-Si system containing Cu. High bondability was achieved by adding both Si and Cu to 6000 system aluminum alloy. High carbon content in steels caused the poor bondability, however, Si and Mn addition to the steel with large amount of carbon was effective to suppress the negative effect of carbon. An incubation time until a reaction layer was formed was recognized at lower bonding temperature and the time depended on additional elements. The reaction rate equation taking into account of an incubation time was newly proposed. This estimated bonding range based on the proposed reaction rate equation shows a good agreement with the obtained experimental results.