The Cu thin films on (111) of Si substrate were prepared by the co-sputtering equipment of ion beam enhance deposition, using the nano-indentation experiments, the elastic moduli and hardnesses of Cu films at different annealing temperature were obtained. The yield strength of Cu film was calculated by FEM model based on the nano-indentation experiments. It is found that yield strength of Cu film is much higher than that of bulk of Cu. And the annealing temperature affects the yield strength of film greatly. From XRD spectrum, it is analyzed that the decrease of yield strength of Cu film results from the change of crystal size and crystal plane.