A simulation of Cu L&agr; and Cu K&agr; diffractions from W/Si multilayers with a local layer‐thickness deviation at various depths of the multilayers has been presented. The results show that the peak intensity decreases rapidly at first while increasing the depth of the local deviation, and then the peak splits into double peaks. But, afterward, the double peaks coincide again. These changes are affected by the ratios (x) of heavy‐atom layer thickness to total period thickness. The intensities of the double peaks are nearly equal whenxis 0.5, but are quite different whenxis 0.22. The changes due to the local deviation are different from those due to random fluctuations or systematic deviations in periods. An explanation has been carried out by the amplitude‐phase diagrams.