Abstract: A model of grade transition is presented for the commercial fluidized bed gas-phase polyethylene production process. For two polyethylene products,the model-based dynamic optimization is used to determine optimal grade changeover policies. The quantity of off-specification product and the time of grade transition can be minimized by optimization of manipulated variable profiles, such as polymerization temperature, the ratio of hydrogen to ethylene, the ratio of co-monomer to ethylene, catalyst feed rate, and bed level. The results show that polymerization temperature, hydrogen and co-monomer are sensitive to the quality of resin. It is always recommended that catalyst feed rate and bed level decrease in order to reduce the grade transition time and the quantity of off-specification product. It is demonstrated that melt index and density are not sufficient to characterize the properties of polymer produced during grade transition. A new performance index, i.e. ratio of melt flow ( MFR ), is included into the objective function, which could restrain the sharp fluctuation of operation variables and narrow the distribution of molecular weight of resin. There are much difference between the transition and reverse transition of grade which indicates the non-linearity of operation.