Abstract: Twin-wire cold metal transfer (CMT) fillet welding was conducted on 3 mm thick 5083 aluminum alloy and 16 mm thick 6063 aluminum alloy. The optimal parameters, including torch angle, welding speed and back wire welding current, were determined by analysis of the weld formation and tensile resistance. The microstructure and hardness of the joint welded with the optimal parameters were studied. The results show that the optimal process parameters were listed as follows:torch angle of 60°, welding speed of 2.0 m·min^-1 and back wire current of 140 A. The obtained weld surface had a smooth transition and no defects such as undercuts and hump; the tensile force per unit length reached the largest value of 548.7 N·mm^-1. The boundary between the fusion zone and the heat affected zone at 5083 aluminum alloy side was distinct, and the impurity phase in the heat affected zone was enriched at the grain boundary. The fusion zone at 6063 aluminum alloy side was narrow, and the weld structure was composed of dendrites and had a strip structure with different grain sizes. The weld zone had the highest hardness, and the fusion zone and heat affected zone were neither hardened nor softened.