Comparative studies have been made on the rheological and mechanical properties of polypropylene(PP)/high density polyethylene (HDPE) binary blends or PP/HDPE/ethylene-propylene-diene terpolymer(EPDM) ternary blends and PP/low density polyethylene(LDPE) binary blends or PP/LDPE/EPDM ternary blends. Morphological analysis has been also performed using scanning electron microscopy. Blends have been prepared by melt mixing in an extruder. The rheological measurement and morphology showed that the interfacial adhesion between PP and HDPE was better than between PP and LDPE, even though both the PP/HDPE and PP/LDPE binary blends were typical incompatible systems. The viscosity of binary blends of PP with HDPE or LDPE increased with the addition of EPDM. The impact strength of the PP/PE binary blends were strongly improved by the addition of the EPDM at the expense of slight loss in yield strength and initial elastic modulus. It was found that the toughening effect by EPDM was more prominent in PP/HDPE blends than PP/LDPE blends, due to the better interfacial adhesion between EPDM and HDPE than EDPM and LDPE.