In this paper a theoretical :model is developed to describe the variations of crystal size and solute concentration with the vertical position in a continuous fluidized bed crystallizer. It is assumed that the liquid phase moves upward through the bed in plug flow and the solid phase in the fluidized bed is perfectly classified. This model would more reflect the actual fluidization behavior of crystals as the variation of bed voidage along the bed height is considered. It allows one to predict the outlet solute concentration and the product crystal size against the total bed height of a fluidized bed crystallizer. The effects of seed feed rate and height/diameter ratio on the performance of a continuous fluidized bed crystallizer are also discussed.