This article describes the impact and dynamic mechanical properties of rubber-modified binary blends of polypropylene (PP). Two conventional elastomers [viz. ethylene vinyl acetate copolymer (EVA) and ethylene propylene diene terpolymer (EPDM)] were used as an impact modifier for PP. It is clearly indicated by the results that EPDM is better than EVA as an impact modifier of PP. Analysis of data of dynamic mechanical properties and impact properties at various compositions of the blends revealed a direct correlation between impact properties and dynamic mechanical loss tangent. The energy dissipation due to viscoelastic relaxation is therefore suggested as a mechanism of impact toughening of PP, in addition to the other commonly known mechanisms of toughening (viz. shear yielding and crazing induced by deformation of rubber-phase domains).