Abstract: A mathematical dynamic model for the fractionator of the Fluidized Catalytic Cracking Unit (FCCU) has been developed. In this model the liquid products of the catalytic cracking are described by four pseudocomponents. Material balance and enthalpy balance are written for each plate. The supplementary equations of this model include plate hydraulics and vapor- liquid equilibrium of pseudocomponents were selected from existing correlations and linearized enthalpy models of pseudocomponents and inert components were put forward by the authors. The results of the simulation experiments show that this dynamic model can predict the dynamic and static responses of each plate when the operating conditions of the fractionator are changed. The predictions of this model fit in with the data of an industrial FCCU fractionator.