Highly filled calcium carbonate (CaCO3)/polypropylene (PP) blends were compounded using an intermeshing corotating twin-screw extruder. The effect of screw configuration, rotation speed, flow rate, filler feed port position and barrel temperature on dispersion of calcium carbonate into polypropylene was investigated. A method based on image analysis of optical transmission micrographs obtained from 25 micron thick films was found to be quick and reliable for evaluating the state of dispersion in the filled systems. The dispersion state affected the mechanical performance of the systems. In particular, falling weight and unnotched Izod impact resistance decreased significantly as dispersion got poorer. Mechanical properties were correlated with the relative volume concentration of agglomerates in the blend.