To realize the nanometer scale thin film thickness traceable measurement, the calibration method of monolayer thickness is studied.In this method the calibrated x-ray diffraction System is used as measurement standard set-up which can be traced to SI units.Firstly, it is based on the theory of the grazing incidence X-ray reflectivity, to analysis peak positions in the measured X-ray reflection intensity curve and the scanning range is from incident angle 0=0.02 ~0.2° to 2~6°.