The fundamental problems in two immiscible polymer blends, such as deformation, break‐up, and coalescence of the dispersed phase, were considered. Instead of formulating a single droplet problem, it was assumed that there is a kind of structure of the interfaces, in which the interfacial area (Q) and its anisotropy (q ij ) are equilibrated due to the competition between flow and interfacial tension. Relaxation mechanisms of the interfaces in heterogeneous systems were phenomenologically considered so that a more general constitutive equation was proposed, which can be used not only for arbitrary volume fractions but also for arbitrary flow fields. Also, the effect of simple shear flow on the morphology of polystyrene (PS)/linear low‐density polyethylene (LLDPE) blends was experimentally investigated. Whereas most works along these lines have been done visually in a flow cell, our samples were quenched after steady shear and their resulting structures were analyzed by scanning electron microscopy. In order to achieve a better understanding of morphological effects on polymer blending processes, the semiphenomenological expressions describing the interface contributions of immiscible polymer blends were formulated and compared with dynamic shear experiments of PS/LLDPE blends.