This work aims to probe the roles of ethylene propylene segmented copolymer (EPS) in impact polypropylene copolymers (IPCs) by rheological and morphological investigations. A series of IPCs with different EPS contents and molecular structures are prepared by an atmosphere-switching polymerization process (ASPP). The Palieme emulsion model is used to describe the relationship between the rheological response to small amplitude oscillatory deformation and the morphology of IPC. It is found that this model describes well the linear viscoelastic responses of IPC, if the role of EPS is taken into account. An increase in the content of EPS and the length of its PP segments leads to a decrease in the size of the ethylene propylene random copolymer (EPR) phase domains and the interfacial tension. These results strongly confirm the role of the EPS as a compatibilizer in the IPC system. The adhesion between the PP matrix and the EPR phase domains is enhanced by the presence of the EPS that is produced in situ during the ASPP. For this reason, ASPP is capable of making IPC with an excellent rigidity toughness balance.