In this study, ternary-phase polypropylene (PP) composites containing an ethylene-octene copolymer (EOR) and calcium carbonate (CaCO3) were investigated. Particular consideration was given to the influence of stearic acid treatment of the filler on the phase morphology and mechanical properties of the composites. In composites containing an uncoated filler, a separate dispersion of the elastomer and filler particles in the PP matrix was observed. The use of filler treated with stearic acid had no effect either on the dispersion or the interaction of the filler and the polymer components. However, the surface-treated filler was found to promote the beta-hexagonal crystallization of PP and gave a composite with lower T-c onset and T-c values. As a consequence, differences in mechanical properties, in particular, impact strength, were exhibited in which calcium carbonate with stearic acid treatment was apparently more effective in increasing the impact strength of the composites in comparison with the composites containing the uncoated filler.