Effect of viscosity ratio (nEPR/nPP), propylene (C3) content of (ethylene-propylene copolymer (EPR), and peroxide/coagent treatment on polypropylene (PP)/EPR (80/20 by weight) melt blends were studied in terms of morphological, rheological, thermal, and mechanical properties. As the viscosity ratio increases from approximately 0.8 to 1.2, domain size increased 9submicron-1.5nm), and the degree of supercooling (T) for crystallization increased (37.4-47.8 C) due to the decreased crystallization temperature (T cc, 122.2-110.8 C). This resulted in larger spherulite size and increased hardness, modulus, and yield strength. With high C3 EPR, total crystallinity (Hf) of PP decreased, together with the mechanical properties, except the impact strength. With peroxide/coagent treatment, the spherulite size significantly decreased. The notched Izod impact strength decreased with increasing viscosity ratio, but significantly increased with high C3 EPR and with peroxide/coagent treatments. The results were interpreted in terms of domain size and shape, chemical affinity between PP and EPR, copolymer formation, and main chain scission of PP.