An approach to the simulation of X-ray diffraction patterns of nanomaterials, which takes into account the contribution of the defect grain-boundary structure described in terms of trapped lattice dislocations (TLD), is developed. As a result of computer simulation, it was found that the parameters of X-ray diffraction peaks considerably depend not only on the density of dislocations but also on the dislocation configuration at grain boundaries. The presence of TLDs results in a deformation-induced broadening of peaks which is primarily determined by glissile grain-boundary dislocations that produce fields of compressive and tensile stresses in the bulk of grains; the resulting shift of the centers of gravity of the peaks is determined by the presence of an excess density of these stresses. Based on the comparison of data on computer simulation with experimental results, we estimated the parameters of the defect structure that were responsible for changes in X-ray diffraction peaks of nanostructured copper produced by severe plastic deformation.