Abstract: The formation process of interface Kirkendall voids is accompanied by the formation and diffusion of vacancies, so the behavior can not be ignored during interfacial diffusion and void formation process. In this study, the diffusion process of Cu-Sn interface and the effect of vacancies on diffusion in Cu/Cu₃ interface were investigated using molecular dynamics simulation. Moreover, the diffusion behaviors of atoms were analyzed, while the vacancy formation energy, diffusion energy barrier and vacancy diffusion activation energy were obtained. By comparison, it was found that the models containing vacancies more easily formed diffusion than the models without vacancies. In addition, the formation energy of Cu vacancy in the Cu crystal was greater than that in Cu₃Sn crystal. The formation energies of Cu₁ vacancy and Cu₂ vacancy were close to each other in Cu₃Sn crystal, and they were both smaller than the formation energy of Sn vacancy. Furthermore, the vacancy diffusion barrier and vacancy diffusion activation energy of the Cu/Cu3Sn interface were calculated. The results showed that the vacancy diffusion activation energy of Sn was higher than that of Cu.