The Cu thin films were deposited on p-type Si (111) substrates by magnetron sputtering at room temperature. The diffusion and interface reaction of Cu/SiO2/Si (111) systems were studied for different annealing temperatures by X-ray diffraction (XRD) and Rutherford backscattering (RBS).We obtained some useful results in the following aspects：The onset temperature of interdiffusion was 450℃ for the Cu/SiO2/Si (111) systems.With the increase of annealing temperature, the interdiffusion was more apparent. There were no copper silicides formed below annealing temperature of 450℃ for the Cu/SiO2/Si (111) systems. The onset temperature of silicification was 500℃. Copper silicides were formed on the samples after annealing at 500℃.