The welding between aluminum alloy and stainless steel is of practical importance for widening the application of aluminum alloys, however, it is difficult to join them using the conventional welding method because of their different physical and metallurgical properties. In this work, inertia friction welding has been used to create joints between LF6 aluminum alloy and stainless steel, and the microstructure and the mechanical properties were analyzed. The results showed that the flash of the joints was mainly caused by friction and extrusion of aluminum alloy during welding, and the microstructure in aluminum alloy side was divided into fine-grain and elongated-grain area. The EDX results displayed that the welded joint was of evident concentration gradient, and a very narrow interface diffusion layer was formed under the function of friction heat and upset force. The fracture mechanics model showed the fracture occurs at the weak area of aluminum alloy side.