The relationship between the evolution of morphology of concentrated immiscible polymer blends and their rheological properties during stress relaxation were investigated by morphological observation on a shearing device. It was discovered that the relaxation time of the droplets immersed in concentrated blend systems increased with the increase of concentration. The rheological properties of concentrated blends were investigated on a rotational rheometer. It was detected that the relaxation time of concentration blend systems was prolonged with the increase of concentration. The increase of the relaxation time of both the droplets and the blend systems with the increase of concentration are related directly to the hydrodynamic interactions among the droplets in concentrated blend systems. A constitutive model for concentrated polymer blend system is proposed on the base of MM model with the concept of effective medium. The environment around a droplet in concentrated blends is treated as an effective medium consisting of other droplets and the continuous phase. The viscosity of effective medium is not only a function of concentration

but also dependent on the morphology of droplet. The new suggested model can reasonably predict the evolution of the phase morphology and the stress relaxation behavior of the concentrated blend systems.