High dispersion and low degree of crystallinity of supported MoS2 nanoparticles have almost excluded the conventional X-ray diffraction (XRD) analysis from a range of physical methods for the characterization of molybdenum-based hydrotreating catalysts. High-resolution transmission electron microscopy (HRTEM) remains a powerful and preferred technique for obtaining information on the dispersion of supported MoS2 nanoparticles and stacking degree of MoS2 slabs. Here, we report a new approach to study the supported MoS2 nanoparticles in catalysts on the basis of XRD data. Alumina-supported MoS2 catalysts were investigated by means of the Debye function analysis (DFA) applied to the XRD data obtained using the conventional laboratory equipment. Through a direct simulation of XRD profiles by the DFA technique, structural information is extracted from both Bragg and diffuse scattering. We demonstrate that it is possible to determine the average size of coherently scattering MoS2 crystallites, crystallite size distribution, as well as average number of stacked layers in the MoS2 particles from the XRD data. Compared to the widely used HRTEM study, the DFA analysis underestimates the MoS2 particle size due to structural defects. The significant discrepancy between the XRD and HRTEM data serves as an indicator of abundant defects causing a multidomain structure of MoS2 particles. It is shown that HRTEM and XRD data complement each other by providing information on the MoS2 dispersion at different levels of particle organization.