Aluminum alloys are materials providing lightness in aviation, shipbuilding, railways and automobiles. Aluminum alloys, which are widely used in automotive industry, are expensive compared to steel, also brought about economic concerns. As a result, hybrid structures made of thin sheet materials, which aluminum can be used with steel, have been an economic solution providing lightness. Therefore, joining of steel with aluminum has become inevitable. Traditional gas welding methods; Thin parts have not been successful due to excessive heat input at the welded joints of different metal alloys such as high strength aluminum alloys and Fe/Al, which have been subjected to precipitation hardening. Excessive heat input causes problems such as deterioration of the mechanical properties of the weld zone, sensitivity to hot cracking, pore formation in the weld metal, and burning and collision, especially in thin sheets. According to the Fe-Al phase diagram, aluminum having a low melting point, Fe, Al solid solutions when contacted with molten steel, and ferric-rich Fe3Al, FeAl and aluminum rich FeAl2, Fe2Al5, FeAl3 and Fe2Al3 intermetallic compounds, making it impossible to combine them with conventional gas welding method. In recent years, research and development efforts of welding equipment manufacturers have led to the emergence of cold metal transfer welding, a new type of low-energy welding thought to be the solution to the joining of thin materials and metal alloys with limited weldability. Cold metal transfer welding which is characterized with low heat input and spatter-free weld, is recommended for joining dissimilar materials. Cold metal transfer welding is a short-circuiting metal transfer. The droplet detachment is assisted by a backward movement of the wire. The wire feeder gives the wire a backdrawing force when the liquid droplet on the wire tip contacts the weld pool; meanwhile, the shortingcircuiting current is reduced to a very low level. Then, the droplet transfers into the weld pool without liquid bridge fracture which happens in conventional arc welding. It indicates that cold metal transfer not only reduces the heat input but also keeps free of spatters because it does not need a high current to fracture the liquid bridge. In this study, overlap welding of AA6061 and galvanized steel sheets was carried out by cold metal transfer method using AlSi5 filler wire. The effect of heat input on mechanical properties and the formation of intermetallic compound (IMC) layer between steel and aluminum were investigated. Mechanical properties of the joints were determined by tensile and hardness tests and also applying nano indentation to intermetallic layer. In order to determine the elastic modulus and hardness of Al-Fe intermetallic phases, a nanoindentation test was applied under a load of 5 mN with Berkovich indenter. Then nanoindentation test was performed and P-h curves were obtained. It has been determined that the intermetallic layer of sample A has higher mechanical properties than sample B. The EDX analysis was found to be higher for silicon B than for sample A. The Al8Fe2Si phase formed in B sample is proved by the results of nano indentation which reduces the intermetallic layer's brittleness. It has been found that the heat input formed during welding acts on the type and thickness of the intermetallic formed between Al / Steel. The hardness of this region is lower than that of sample A because of grain refinement in the steel main material of B sample. In both cases, the lowest hardness values were determined on the weld metal. When the tensile strength of the welded samples is examined, the maximum tensile strength and the unit elongation values are increased by increasing the heat input (provided that thickness of the intermetallic is less than 10 μm). SEM-EDX analysis revealed the formation of FeAl and Al3Fe in sample A, and Al3Fe and Al8Fe2Si intermetallics in sample B. The hardness and modulus of elasticity of the intermetallic layer formed at A were higher than the B sample. As a result; by cold metal transfer welding, It can be successfully combined with Al-Steel sheets by using appropriate welding parameters and intermetallic layer thickness control, can prevent the parts from heating up more adversely affecting the mechanical properties of the joints compared to other arc welding methods.