Abstract: The constituent and the structure of the interface layer between the low temperature deposited diamond films and the single crystal silicon substrate was studed by X-ray diffraction (including small angle diffraction) technique. It was found that the interface layer for diamond film deposited at 700℃ was α-SiC. At lower temperature range (580-290)℃, the interface layer was composed of α-SiC and SiO₂. Crystal structure of α-SiC and SiO₂, and the d-spacings of the diamond films were discussed.