A complex of trivalent lanthanum stearate with stearin (REC) was used as a surface modifier to treat calcium carbonate, and its efficacy was assessed. The measurement of the viscosity of a CaCO3/liquid paraffin suspension demonstrated that REC treatment could considerably improve the dispersion of filler in nonpolar media. The FTIR spectrum analysis revealed that the REC bonded to the CaCO3 surface not only by physical absorption but also by a chemical reaction. The REC could change the surface properties of CaCO3, resulting in greater hydrophobicity of the surface and enhanced compatibility with nonpolar matrices. The mechanical properties and melt flow rate (MFR) examination of polypropylene (PP)/CaCO3 composites showed that REC treatment remarkably enhanced the toughness and MFR of composites at higher filler loading. The fractured surface observation with SEM showed improved filler dispersion and enhanced interfacial adhesion between the filler and matrix in PP composites filled with REC-treated CaCO3. All these improvements demonstrated that REC is an effective surface modifier of CaCO3.