Calcium carbonate ($CaCO_3$) with 15~40 nm average diameter was compounded with polypropylene (PP) in a melted state to form composite materials. To enhance the dispersibility in PP, the surface of the $CaCO_3$ was coated with hydrophobic stearic acid (SA). The SA-coated $CaCO_3$ (SA-$CaCO_3$) showed hydrophobic nature, which resulted in rapid precipitation in water. When melt-compounded with PP, SA-$CaCO_3$ was uniformly distributed throughout the PP matrix, while the uncoated $CaCO_3$ formed aggregated structures in the PP. At up to 5 wt% of SA-$CaCO_3$ content, the composites showed well-dispersed structure, although some level of aggregation was observed in the composites with 10 wt% of SA-$CaCO_3$. The dispersibility improved with a repeated process of compounding. The melting and crystallization temperatures of PP increased with incorporation of $CaCO_3$. The enthalpy of the transitions also increased and the full-width of half maximum of the crystallization peak decreased. However, SA-coating did not produce additional effects on the crystallization behavior of PP. These results showed that the $CaCO_3$ acted as crystal nuclei.