Abstract: 1.5 mm thick 5 A06 aluminum alloy and 3 mm thick galvanized Q235 steel were lap joined by TIG welding-brazing with Al Si5 filler metal. Dissimilar metals joints were annealed at 280 ℃ for 30 min and the effect of post-weld annealing heat treatment on microstructure and mechanical properties of joints was investigated. The results show that the welding of dissimilar metals between aluminum and steel can be realized by TIG welding-brazing and the intermetallic compounds layer is inevitably formed at the interface between aluminum and steel. The intermetallic compounds layer is composed of Al₈Fe₂ Si,a few Al,Fe,Siphases and a few Al₈Fe₂ Si phases and a few Al₁₃Fe₄ phases with a total thickness of 4-5 μm. The tensile strength of as-welded joints reaches 163 MPa,and the fracture occurred in weld seam. After the joint being annealed at 280 ℃ for 30 min,the microstructure of intermetallic compounds layer doesn’t bring out obvious changes,which consists of Al₈Fe₂ Si,a few Al,Fe,Si phases and a few Al₈Fe₂ Si phases. The thickness of compound layer in annealed joint increases to 9-10 μm.The tensile strength of annealed joint increases to 185 MPa,and the fracture occurs in aluminum base metal.