A simple formula for calculating the interfacial roughness of multilayer by using the small angle X ray diffraction curves of the samples is given. The Mo/Si multilayer is prepared by using the magnetron sputtering, and the small X ray diffraction spectrum for the multilayer is measured by using X ray with the wavelength of 0.154 nm. The interfacial roughness are given by the combination of the formula with the small angle X ray diffraction spectrum, and fitting the measured spectrum to the theoretical diffraction spectrum respectively. The results show that the calculated data of the interfacial roughness from the formula is closer to that given by fitting curve, which equals the real roughness of Mo/Si multilayer.