IFW method has the potential of being ideal technology to weld dissimilar materials, aluminum and stainless steel were welded by inertia friction, and the morphology, microstructure, interfacial composition and mechanical properties of Al/steel joints were investigated. Results showed that a thin IMC reaction layer was found at the welding interface in the joint, and the IMC was consisted of Al, Fe and high concentration Si. The microstructure of joint contained weld nugget zone, fully dynamic recrystallized zone, thermal mechanically affected zone and heat affected zone. The grain of fully dynamic recrystallized zone (FDRZ) was below 0.1 μm, and the average width of FDRZ in the joint was about 5 μm. The maximum hardness was in the FDRZ, and the maximum value was 395.8 HV in the joint. Tensile strength of joint was influenced by the rotational speed. When the rotational speed was 1100 rpm, the joint reached to the maximum tensile strength of 323 MPa. The high tensile strength reason should be related to the thickness of IMC at the weld interface.