In the studies of residual stresses of surface materials by using X-ray, in-depth distribution of the strain in the surface layer is necessary to be analyzed. We, therefore, characterized the refractive index of the surface layer materials with complex refractive index, which changes continuously in depth, and derived the X-ray intensity propagating during the surface layer materials. We applied this analyzing method to the experimental results of X-ray diffraction at small glancing angle incidence, and obtained the depth profile of the strain in the surface layer. The derived analyzing method can be applied to the residual stress distribution analysis of the surface layer materials of which densities change continuously in depth as multi thin films, compound plating layers. (c) 2004 Published by Elsevier B.V.