The mechanism of liquid mixing by solid particles in fluidised bed has been considered and a mathematical model of dispersion in liquid phase was proposed. The model takes the time scale of the random motions of fluids elements into account and mathematically is based on a partial differential equation using the probability distribution function. For the cases of an instantaneous and continuous point source simple integral formulas were found which allow to calculate the tracer particle distribution at a short distance from the source. The model was compared with the previous models, particularly with the diffusion model and was then used to approximate the experimental results showing the variation of the mean radial square displacements of the discrete tracer particles with the distance from a continuous point source located in a liquid fluidized bed. The effect of void fraction on the relaxation time and the dispersion coefficient in the liquid phase was determined.