Abstract: The paper introduces the principle of measurement of super-smooth surface by grazing incidence X-ray scattering (GXRS) method and the experimental facility based on an improved X-ray diffraction device. The scattering diagrams measured from three wafers with different roughness are treated by the first-order vector perturbation theory (FOPT) and the generalized Harvey-Shack theory, respectively. The results indicate that the calculated power spectrum density (PSD) functions are in a good agreement with the results obtained from atomic force microscope. The effects of the slit width of detector and the divergence of incident X-ray on the measurement are also analyzed. It is concluded that the measurement error decreases exponentially with reducing the slit width and the incident divergence,