A mathematical model allowing us to calculate the rising and falling velocities of a single particle in the liquid fluidized bed formed by particles of different sizes and densities was proposed. The model was examined by measurements of the falling velocity of spheres with greater diameter than the diameter of the bed spheres. The experimental results are in agreement with the results predicted on the basis of the mathematical model. The methods used to calculate the classification velocity of particles in beds containing mixtures of different particles were discussed.