A new method which can simultaneously determine the constituent and thickness of the alloy films using the X-ray EDS has been developed. The constituent is determined by the ratios of the kα intensties of elements in the film. The thickness of the film deposited on a NaCl substrate is measured by the attenuation of the ratios of Na kα intensity/Cl kα intensity. It is not necessary to use bulk standards of pure elements. This method has been applied to Cu-Si alloy films of different thickness deposited on NaCl substrate. Under various experimental conditions, the constituent and thickness data are determined satisfactorily.