Abstract: An experimental investigation of ultrasonic welding of 1mm thick 6061 aluminum alloy and pure copper was conducted in this research. Weld appearance,plastic deformation and atomic diffusion in the weld zone under different welding energy input have been investigated. The results showed that swirl-like plastic deformation occurs at the interfaces due to the high-frequency ultrasonic vibration,which led to a mechanical interlocking between materials. The plastic deformation was too small to gain a qualified joint under a lower energy input,while void defect was likely to appear if the energy was excessively high. Moreover,intermetallic compound (IMC) at the interface of the workpieces was also observed through the SEM and EDS analysis. A 0.5 μm-thick IMC layer with the dominant phase of Al₄Cu₉ was found in 2000 J welding energy joint.