Abstract: Although high-power ultrasonic welding(USW) is better suitable for joining dissimilar metals, there is few studies on application of this welding method in automobile lightweight. The interface performances and joint properties, including interface temperature rise, intermetallic compound (IMC) growth, joint strength and fracture morphology, in 6061-T6 aluminum alloy dissimilar high-power USW are studied. In USW of Steel/Al combination, at 1 s welding time, 1.98 kN normal force, and 27 µm amplitude, a continuous IMC layer with a thickness of 1.7 µm occurs at welding interface. The IMC is primarily composed of FeAl₃ and Fe₂Al₅. About half of the ultrasonic power is converted into interfacial energy. The welding zone is significantly larger than the sonotrode tip area. High plastic deformation and obvious mechanical interlocking occur at the interface. The obtained highest lap-shear strength of Steel/Al joint were 3.95 kN. The tensile failure behavior of the joint is interface failure, and the fracture mode of the joint is ductile-brittle hybrid fracture. In USW of Cu/Al combination, at 0.54 s time, 1.45 kN normal force, and 25 µm amplitude, the IMC layer with thickness of 1.95 µm at the interface is primarily composed of Al₂Cu; the obtained highest lap-shear strength of Cu/Al joint is 3.20 kN, and the tensile failure behavior of the joint is pull-out failure. The relationship between IMC thickness and joint strength shows that proper IMC layer thickness can improve the joints strength.