Abstract: Butt welding of dissimilar metal between Al alloy LF2 and carbon steel Q235 with the middle transition metal of Cu was carried out by electron beam welding.The microstructures, elements distribution in the joint were investigated by means of optical micrography, SEM and EDX. The result showed that the macrostructure in the joint can be divided into three zone.One is Fe based solid solution with a small amount of intermetallic compounds at the weld zone near to steel side.One is Al based solid solution with a definite store of Fe -Al and Al -Cu intermetallic compounds at the weld zone near to aluminium alloy.The other is mixed intermetallic compounds zone which consists of the several kinds of Fe - Al and Al-Cu intermetallic compounds at the middle weld, and its distribution is layered and banded.The analy sis indicated that more intermetallic compounds were made in the weld although the transition metal Cu was used, particularly the multiple bedded and zonal intermetallic compounds formed and distributed in the middle weld are the main factor to influence the strength of the joint.Based on the analysis on microstructure of the joint, the physical model was founded to describe the forming process of dissimilar metal joint between Al alloy LF2 and carbon steel Q235 with transition metal Cu by EWB.