The particle size distribution in a polydispersed mixture significantly affects the technological behaviour of the mixture. Generally, mathematical procedures are required to estimate the particle size distribution or concentration distribution in a sorting process of such a mixture. A theoretical model is presented in this study to emphasize the major system parameters affecting the particle size distribution under hydrodynamic and gravitational effects. First, an analytical relationship is derived to represent the concentration profile in a sorting column during the classification of polydispersed mixtures. Then, the Focker-Planck equation is used to obtain the distribution function depending on the particle size. Some of the parameters of these analytical relationships are evaluated by comparing the equations with experimental data. The results indicated that the model predictions and the experimental data are in good agreement.