Abstract: It is very important and difficult to obtain accurate layer thickness of super thin films for further research of its optic and electronic properties. Three measuring methods were used to obtain the layer thickness of super thin films, which are reflectance curve fitting in soft X-ray region method, Bragg diffraction equation method and reflectance Fourier transform method. The reflectance of super thin films were measured using soft X-ray of synchrotron radiation, and X-ray diffraction were also measured at the same time. Two samples, single layer of W film based on silicon substrate and bilayer of C/W film based on silicon substrate, were used to test the accuracy of the results obtained from the three methods. The single layer W film’s the thicknesses measured by the three methods are