Abstract: Real time dynamic measurements on the vibration amplitude of the sonotrode, the upper and lower copper sheet were performed using laser displacement sensor during ultrasonic welding of copper sheets. The welding process was analyzed on the basis of the relative motion among three parts and the microcosmic evolution of fracture surface. The results showed that during the welding process the amplitude of sonotrode changed little and that of the lower sheet increase gradually. Meanwhile, the amplitude variations of the upper sheet progess is increased at first then decreased and increased again. In the amplitude increase stage, a quantity of friction heat were generated owing to the relative motion between the two welded sheets. Besides, the microjoining appeared at the welding interface, In the amplitude decrease stage, the relative motion between the upper sheet and the sonotrode increased rapidly. It prompted the occurrence of the severe plastic deformation and the effective bonding area increased rapidly. Finally, the upper and lower sheets vibrated at the same amplitude. A large number of dimples were observed on the fracture surface.