Abstract: A 3-D thermal-mechanical coupled finite element model of ultrasonic metal welding of dissimilar alloys is developed. The friction heat flux and the deformation heat flux are related to different level of coupon’s vibration amplitude. The effects of ultrasonic softening on temperature distribution, stress distribution and tool indentation depths are included in the analysis of the welding process. The results show that the bulk temperature is lower than the melting temperature of the metals, the maximum temperature occurred at the center of the contact interface between the sonotrode tip and the upper copper specimen and the peak level of the welding interface temperature occurred at the center area. It is also shown that the sonotrode tip completely sinks into copper in welding, while the anvil tip’s indentation depths do not reach its maximum. The proposed model is validated and verified by comparing with welding crosssection and a point temperature using experimental physical test. The presented model is capable of predicting and explaining the relationship between ultrasound soften, mechanical field and thermal field during welding process.