Various chemical engineering processes, such as separation, purification or classification, apply hydrocyclones, either stand-alone or as cascaded, multi-level systems. However, calculation of transfer processes in hydrocylcone classifiers remains difficult due to the complex behavior of the dispersed particles. Here, an analytical model of particle classification process of liquid-solid polydisperse systems in cylinder-cone hydrocyclone classifiers of small sizes has been developed. The applicability of a probability and statistical calculation method by using diffusion-type equations and, in particular, the Fokker-Planck-Kolmogorov equation, is theoretically shown.