In recent years, dissimilar joining between steel and aluminium alloy is strongly expected in especially automotive industries to get weight–saving of car body by hybrid–structure of these two metals, and to contribute to fuel efficiency and futher CO2 reduction. In this investigation, we discuss the dissimilar weldability between low carbon steel and aluminium alloy further more by focusing formation behaviors of intermetallic compound (IMC) under various joining processes. Namely, in order to change heat input conditions, GTAW, resistance spot welding and friction stir spot welding were applied. Microstructure observation, EPMA analysis of IMC, vickers hardness test and tensile shear test in welds were performed. As a result, the remarkable volume of IMC and cracks were observed in GTAW. On the other hand, in resistance spot welding and friction stir spot welding, namely, in the case of smaller heat input conditions, formation of IMC(indentified as Fe3Al, FeAl, FeAl3 in resistance spot welding) and cracks in welds were inhibited and mechanical properties were improved. And we got a knowledge of possibility of dissimilar joining between steel and aluminium alloy by proper heat input control.