Abstract: In this paper, the theory of Markov chain is used to discuss the behavior of bubbles in G-L-S fluidized bed. A generalized stochastic model is proposed. On the basis oi thermodynamics, the model parameters were analysed and discussed to obtain a simplified stochastic model of bubble size distribution. In addition, the distribution of bubble size in a twodimensional G-L-S three-phase fluidized bed were measured by using conductivity probe method and micro-computer sampling technique. The calculated values with the stochastic model were very close to the experimental data.