In order to understand the mixing in polymer extrusion, the morphology of PS/LDPE blends compatibilized with SEES was simulated based on the Lee and Park model. Experiments were performed in a co-rotating twin screw extruder and a Haake mixer, and the morphology of the compatibilized blends of various compositions was compared with the non-compatibilized ones. The size of the dispersed phase of the compatibilized blend was smaller than the noncompatibilized one, and the blend of the PE matrix had a smaller size in the dispersed phase than the non-compatibilized one. Simulation results were agreed well with experiments, and the Lee and Park model could be applied in the extrusion analysis of compatibilized polymer blends.