Polypropylene (PP)/calcium carbonate nanoparticles (nano-CaCO3) composites were prepared using a two-stage modular extruder with the assistance of supercritical carbon dioxide (sc-CO2). The crystallization and melting behavior, crystal forms, and crystal morphology were investigated by differential scanning calorimetry, wide-angle X-ray diffraction, and polarized optical microscopy. The loading of sc-CO2 injected into the extruder was optimized to be 2.0 phr. At this optimized loading, nano-CaCO3 particles were homogeneously dispersed in PP matrix. The addition of nano-CaCO3 particles has little impact on the melting behavior of PP, but exerts significant influence on the crystalline morphology of PP/nano-CaCO3 composites. At the optimized loading of sc-CO2, the PP/nano-CaCO3 composite has the highest impact strength due to the best dispersion of nano-CaCO3 particles in the PP matrix.