The Cu films were deposited on P type Si (111) substrates by ionized cluster beam (ICB) technique. The interface reaction and atomic diffusion of Cu/Si (111) system were studied at different annealing temperatures by X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Some significant results were obtained: For the Cu/Si (111) samples prepared by ionized cluster beams at Va=3 kV, the interdiffusion of Cu and Si atoms occurred in the as deposited samples. The RBS spectra features were changed with a very small extent with increasing the annealing temperature. There are no copper-silicide phases observed by XRD before and after being annealed at different temperatures. The reason may be that there is a thermally stable interface between Cu films and Si substrates formed.