A theoretical model for predicting morphology evolution of immiscible polymer blends in a corotating twin-screw extruder was described with a new blend＇s constitutive equation (the Lee and Park model). This theory describes the overall morphology of immiscible binary blends in terms of an interfacial area and its anisotropy, which has several advantages such as no limitations of flow type and applicability for any domain shapes and compositions. In this study, the classical algorithm has been adopted to find the pressure profile and the degree of fill along the screw axis for the modular-type twin-screw extruder. It is based on the screw characteristic curves to obtain a pressure gradient at a given flow rate and backward calculation of the pressure profile starting from the die. The partially filled region was also included to describe properly the morphology evolution. Considering the flow characteristics of this region, we assumed that the apparent shear rate was proportional to the degree of fill. The overall model predictions for the various operating conditions are compared with the experimental results. Therefore, it may be said that the new constitutive equation is useful to describe the morphology behavior not only for the twin-screw extruder but also for the other polymer processing equipment.