Abstract: The dissimilar metals of copper and stainless steel were jointed by friction stir lap welding (FSW) and no internal defects were obtained in the lap joints with good appearance. The microstructures and hardness of welded joint were investigated by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy disperse spectroscopy (EDS). The results show that the onion ring structure of stainless steel and Cu was observed in the welded nugget zone (SZ), which was more uniform in advancing side (AS) compared with retreating side (RS). New intercalations were observed in the layered structure and subsequently confirmed as NiCu₄, for which untypically high hardness values were registered. The hardness of the welded joints in the welded nugget zone was significantly higher than that of the matrix, which was higher in the advancing side.