Abstract: Among the available techniques for the characterization of crystalline nanoparticles (<100nm), X-ray diffraction (XRD) has undoubted advantages and specific qualities. A fast and reliable determination of particle shape and size distribution as well as lattice defect type and content is possible through the use of the recently proposed Whole Powder Pattern Modelling of the X-ray diffraction pattern. The quality and statistical reliability of the results, given in terms of physical quantities such as a size distribution curve, content of dislocations, stacking faults, antiphase boundaries (if any) etc., pairs and in most cases surpasses that of high resolution Transmission Electron Microscopy. As an example, a ball-milled fluorite powder will be analysed and results of traditional methods will be compared with those obtained via WPPM. The weakness of largely used simplified formulae such as the Scherrer formula or the Williamson-Hall method will be thus pointed out