Abstract: X-ray reflectivity measurement using synchrotron radiation is a accurate and easy method to determine the structure of a thin film, compared with other experimental methods. X-ray reflectivity investigation using synchrotron radiation were performed on Cr/SiO₂ thin film system. Reflectivity curves with good S/N ratio were collected. The thickness of Cr film and the roughness of the interface between Cr and SiO₂ substrate were obtained by Fourier analysis and least-square fit of the experimental data. It is shown that the present method is effective for the determination of interfacial roughness.