Fluidized beds containing solid particles of a wide size distribution is of significant practical importance. In such systems, the overall behavior depends on the opposing effect of mixing and classification. In the present work, the mixing and segregation behavior of liquid fluidized beds containing particles of different sizes is described mathematically. Particle size distribution (PSD) is studied in a glass column of 5 cm internal diameter and 250 cm length. Ion exchange resins were used as a solid phase with a particle size range of 50 to 650 mu m. The bed was fluidized at constant and low water flow velocity and the particle size measurement was carried out at different locations over the column length by analytical scanning electron microscope. Particle size fractionation data obtained by Malvern Mastersizer-2000, version 5.4, was utilized in the solution of developed model equations to obtain PSD. It is apparent that the mixing model along with the classification model represents better results than any other model given by various researchers in the literature. The proposed model is in good agreement with the PSD data given by Malvern particle size analyzer.