The melt blending of polymers is an effective and practical way to produce new materials for specific application. Properties of polymer blends depend strongly on their morphology which can be controlled and quantified by rheological properties of the components. In the present work, rheological properties of PP/HDPE blend were investigated at linear and nonlinear viscoelastic range. Relaxation of the step shear strain and morphology were studied at various HDPE concentrations as disperse phase and EPR amounts as a compatibilizer. The blends showed three relaxation stages; a first fast relaxation which was attributed to the relaxation of the components, second one with slower rate, named plateau, which related to droplets relaxation, and the last one with higher relaxation rate. The results showed that the length of the plateau increased with increasing of HDPE concentration, while it decreased with increasing of the compatibilizer content. These behaviors are related to the droplets size, as the smaller droplets relax faster. In the plateau region, blends showed higher relaxation modulus than the components which is concerned to the droplets and it decreased with compatibilizer concentration. It can be concluded that smaller droplets not only relax faster but also cause lower relaxation modulus. The compatibilized blends showed higher relaxation rate at the third region that can be related to the smaller droplet and narrower droplets size distribution which is confirmed by SEM micrographs. This behavior has seen both in linear and non linear viscoelastic range. Keywords: Rheological Behavior, Stress relaxation, Polyolefin blends