A method for calibrating the spectroscopic ellipsometry by X-ray reflectivity is presented. As an indirect method，spectroscopic ellipsometry does not have the traceability because the measured film thickness is dependent on its optical constant. In contrast，at the grazing angle，the X-ray reflectivity can be used to measure the absolute thickness of thin film with sub-nanometer precision，and is independent of the optical constant. Five SiO2 films were deposited on the substrate of single crystal silicon with the thickness of 2 nm，18 nm，34 nm，61 nm，and 170 nm，respectively. The results of spectroscopic ellipsometry and X-ray reflectivity were well fitted by a liner equation with slope of 1013±0013 and its intercept was set to zero，which means this calibration method is valid for spectroscopic ellipsometry in the determination of the thickness of thin films.