Molecular dynamics (MD) simulation was used to investigate the vacancy trap-ping during the solidification and annealing processes of Cu₅₀Ni₅₀ at different temperatures. The results showed that the vacancy trapping effect was very evident and the vacancy concentration significantly increased with the decrease in temperature, opposite to the law that the equilibrium concentration changes with temperature. There was a characteristic temperature T ^∗ for which the solidification rate rapidly increased with the decrease of temperature at shallow undercooling when T > T ^∗ but decreased slowly at deep undercooling when T < T ^∗, indicating that the vacancy concentration was not a monotonic function of the interface moving rate. In addition, as an alloy constituent, Cu was found to be a better vacancy atom (VA) than Ni.