The compounding of calcium carbonate filled polypropylene (PP) is discussed with reference to a single-screw extruder and variants of mixing sections. The mixing section on the screw is exchangeable, and two dispersive mixing elements, namely the Zorro and the Maddock elements, were used. The calcium carbonate was surface treated with a liquid titanate coupling agent (LICA12). The impact strength was measured by a notched Izod impact tester with specimens having a U-shaped sharp notch. The fracture toughness for the PP homopolymer and the filled composites was determined using fracture mechanics principles. The results were correlated with the state of dispersion of the calcium carbonate filler. The effects of filler concentration and surface treatment were examined as well. Correlation between state of dispersion and impact properties for calcium carbonate filled PP was obtained. We also investigated the effect of various mixing elements on the state of dispersion. The experimental results indicate that good dispersion would improve the impact properties of the polymer matrix, but only at moderate filler loading.