This study was carried out to investigate the influences of compounding process and surface treatment on calcium carbonate (CaCO3) filled polypropylene. The compounding process is discussed with reference to a twin-screw extruder and an internal mixer. The calcium carbonate filler was surface-treated with a liquid titanate coupling agent (LICA 12) and stearic acid. Composites of different weight fractions were prepared by both compounding processes, and their impact properties were evaluated. The notched Izod impact strength increased with CaCO3 content up to a maximum at about 10 vol%, and then decreased. Surface treatment of CaCO3 filler generally yielded composites of higher impact strength than untreated system. Though LICA 12 was more effective than stearic acid in modifying the filler, the low-cost stearic acid proved to be more effective when dealing with the impact properties of composites. Moreover, the composites from a Brabender Plasti-corder exhibited better gross uniformity than that from the twin-screw extruder. However, good filler dispersion and uniform microscopic morphology, as revealed by SEM microscopy, was observed in the samples from the twin-screw extruder. Polym. Compos. 25:451–460, 2004. © 2004 Society of Plastics Engineers.