Abstract: Aluminum alloy A5052 and stainless steel SUS304 were welded by resistance spot welding with a cover plate. The welding interface region of the joint was observed with electron microscopy, and the microstructure and distribution of the reaction products were analyzed as well. The results reveal that a serration reaction layer consisting of Fe₂Al₅ and FeAl₃ forms in the welding interface and the reaction layer thickness varies with the welding current and the position at the welding interface. Moreover, the reaction blocks in aluminum alloy near the welding interface were observed, which were estimated as a hexagonal AlFeCr having a=2.451 nm and c=0.758 nm based on analysis of selected area electron diffraction patterns.