The Cu thin films have been deposited on Si(100) substrates by magnetron sputtering at room temperature. The samples were heat treated by conventional thermal annealing in different temperatures: 230°C, 350°C, 450°C and 500°C. The interface reaction and atomic diffusion of the Cu films and Si substrates between as-deposited and as-annealed at different temperatures are investigated by means of Rutherford backscattering spectrometry(RBS) and X-ray diffraction(XRD). Some significant results are obtained on the following aspects: (1) According to RBS, as-deposited Cu/Si(100) samples are not found interdiffusion, and the onset temperature of interdiffusion is 230°C. With the increase of temperature, the interdiffusion becomes more apparent. (2) After annealing at 230°C, the XRD results of the samples showed formation of Cu3Si(300). As the annealing temperature increases, the other copper-silicide phases are formed. The main copper-silicide phase is Cu3Si(300) after annealing at 500°C. It means that Cu3Si is a reliable copper-silicide in a wide range for the Cu/Si(100) interface.