Abstract: Based on the population balance theory, a partial mixing series model, which integrates the kinetics of crystal growth with the hydrodynamics of characterization of the geometric properties and operating parameters of the device, was developed to describe the behaviour of three - phase fluidized bed crystallizer. With this model, the concentration distribution of the solution, size distribution of the crystals and the mean size of the products were estimated. The calculated results agreed well with the experimental data for sucrose crystallization in a three - phase fluidized bed. The fact indicates that the model is quite reasonable, practical and capable of providing a basis for the design and scale up of three - phase crystallization systems.