Against the background of the required weight reduction in transportation through lightweight construction, the application of hybrid structures, where aluminum alloy and steel are jointed together, has a high technical and economical potential. But jointing of material combinations of aluminum alloy and steel is problematic by fusion welding since brittle intermetallic compounds (IMCs) are formed between aluminum alloy and steel. Nowadays, tungsten inert gas (TIG) welding–brazing offers a great potential for aluminum alloy and steel jointing. In this process, the sheet and filler metal are heated or melted by TIG heat, and the joint has a dual characteristic: in aluminum alloy side it is a welding joint, while in steel side it is a brazing joint. However, in the dynamic heating process, the heating temperature changes so quickly and the reaction time between the liquid filler metal and solid steel is so short that it is more difficult to control the IMC layer’s growth, predominantly its thickness and microstructures. Most of past reports about the brazing of aluminum alloy and steel indicate Al–Fe binary IMC layers, e.g., Fe₂Al₅ and FeAl₃, formed in the brazing joint, which are detrimental to the mechanical properties of the joint. Si additions are used to limit the growth of the brittle Al–Fe IMC layer between aluminum alloy and steel by replacing Al–Fe phases with less detrimental Al–Fe–Si phases in aluminizing and furnace brazing of aluminum alloy and steel. By now, there have been few reports of investigating the interfacial layer of TIG welding–brazing joint of aluminum alloy and stainless steel. In this paper, a butt TIG welding–brazing joint of aluminum alloy/stainless steel was formed using Al–Si eutectic filler wire with modified Noclock flux precoated on a steel surface. The microstructure characteristics of the welded seam–steel interfacial layer were analyzed by OM, SEM and EDS and its mechanical properties were measured by dynamic ultra–microhardness tester and SEM in situ tensile tester. The results show that a nonuniform and sawtooth IMC layer forms at the seam–steel interface and its thickness changes from 4 to 9 μm, less than the maximum permissible value (about 10 μm). The interfacial layer is composed of two types of IMC layers, which are τ₅ IMC layer on the seam side  and θ+η+τ₅ IMC layer on the steel side. τ₅ phase forms preceding θ+η+τ₅ due to its lower growth energy than Al–Fe phases and the primary τ₅ layer inhibits the growth of rough dendritic θ+η+τ₅ phases. The ultra–microhardness test results show the microhardnesses of  θτ₅ and θ+η+τ₅ layers reach HV1025 and HV835, respectively. Indentation cracking of τ₅ layer at higher loads indicates that τ₅ is a type of hard brittle phase. SEM in situ tensile test results confirm that cracking initiates from θ+η phases and then fracture rapidly generates along θ+η+τ₅ layer while suffering external force. The tensile strength of IMC layer reaches 120 MPa.