The evaluation of filler dispersion in compounding machines is an important part of their design. Calcium carbonate-filled polypropylene (PP) was used as a model compound to study filler dispersion. The evolution of dispersion was followed in a twin-screw extruder where several types of mixing sections were evaluated. Reverse kneading blocks were found to be very efficient for breakup of agglomerates. Depending on extrusion conditions, agglomeration was observed even after the matrix was melted. To study dispersion in a single-screw extruder, Maddock and reverse Maddock mixing elements were tried. A quantitative evaluation of the dispersion state allowed a better understanding of nucleation in the PP/CaCO3 matrix. A significant content of the beta-phase of polypropylene was observed when the agglomerate size was relatively small. The level of shear stress was also important for the formation of the beta-phase of PP. The quantification of dispersion was mainly evaluated from micrographs of samples obtained by reflected light microscopy in conjunction with image analysis. The characterization of beta-spherulites was carried out using polarized light microscopy and scanning electron microscopy.