The GexSi1-x/Si superlattice with 23 periods was grown by molecular beam epitaxy. The X-ray diffraction pattern was measured using a computer-controlled X-ray diffractometer with Cu Ka radiation. Interference peaks due to the superlattice structure were observed up to the 13th order. The superlattice period and the Ge average composition can be determined from the interference peak angles based on a modified Bragg's law, which was derived by including the X-ray refraction at the superlattice surface and interfaces. The structural parameter can be determined by analyzing the X-ray diffraction pattern based on the optical multilayer reflection theory. The intensity ratio of the 2nd peak to the 1st peak is quite sensitive to the variation of the thickness ratio of the two components. By comparing the calculated value of I2/I1 to the experiment, we can determine the thicknesses of Si and GexSi1-x layers, and finallv x. The diffraction pattern calculated using the optical mutilayer reflection theory was in accordance with the measured ones.