The morphology and mechanical behavior of ternary polypropylene/ethylene-octene copolymer/calcium carbonate composites have been investigated as a function of elastomer and filler contents. A separate dispersion of phases was observed. Increasing concentration of EOC and CaCO3 promotes a decrease of matrix ligament thickness between elastomer particles resulting in a huge increase of the toughness. The stiff filler used in this study provided the unusual additional benefit of substantially increasing in Young's and flexural modulus of the composites.