The aim of this work was to study the effect of surface-modified calcium carbonate (CaCO₃) nanoparticles in the mechanical properties and crystallization behavior of polypropylene (PP) homopolymer. Pimelic acid (Pa) was used as a surface modifier for nano-CaCO₃ (nCC). Three compositions of PP/nCC composites were prepared in a corotational twin-screw extruder machine with nCC content of 5, 10, 15, and 20 wt%. The results of scanning electron microscopy showed that the Pa treatment enhanced the interfacial adhesion between the filler and the matrix, indicating the improvement in the compatibility between PP and nCC. The measurements of the mechanical properties showed that the elastic modulus (E_c) and impact strength (S_IC) of the composites increased at first and then decreased with the addition of fillers, and the tensile yield stress (_yc) was reduced at the same time. The crystallization properties of virgin PP and its composites were studied with differential scanning calorimetry. The results demonstrated that, in comparison with virgin PP and PP/nCC, the addition of the Pa-treated nCC fillers led to a higher crystallization temperature, and nucleation and crystallization were improved at the same time.