Effect of viscosity ratio (eta(EPR)/eta(PP)), propylene (C-3) 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 (submicron-1.5 mu m), and the degree of supercooling (Delta T) for crystallization increased (37.4-47.8 degrees C) due to the decreased crystallization temperature (T-cc 122.2-110.8 degrees C). This resulted in larger spherulite size and increased hardness, modulus, and yield strength. With high C-3 EPR, total crystallinity (Delta H-f) 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 C-3 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.