A new grafting method was developed to incorporate maleic anhydride directly onto solid-state polypropylene powders. Maleic anhydride grafts altered the nonpolar characteristics of polypropylene so that much better mixing was achieved in blends and composites of polypropylene with many other polymers and fillers. Maleic anhydride was grafted onto polypropylene by the peroxide-catalyzed swell grafting method, with a maximum extent of grafting of 4.60%. Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, tensile testing,and impact testing were used to characterize the isotactic polypropylene (iPP), maleic anhydride grafted polypropylene (MAH-g-iPP), and (isotactic polypropylene)/(calcium carbonate) composites (iPP/CaCO3). The crystallinity and heat of fusion of the MAH-g-iPP decreased as the extent of grafting increased. The mechanical properties of the CaCO3 filled polypropylene were improved by adding MAH-g-iPP as a compatibilizing agent. The dispersion of the fillers in the polymer matrix and the adhesion between the CaCO3 particles and the polymer matrix were improved by adding the compatibilizer.