Polypropylene (PP) and calcium carbonate nanocomposites were prepared by melt mixing in a Haake mixer. The average primary particle size of the CaCO3 nanoparticles was measured to be about 44 nm. The dispersion of the CaCO3 nanoparticles in PP was good for filler content below 9.2 vol%. Differential scanning calorimetry (DSC) results indicated that the CaCO3 nanoparticles are a very effective nucleating agent for PP. Tensile tests showed that the modulus of the nanocomposites increased by approximately 85010, while the ultimate stress and strain, as well as yield stress and strain were not much affected by the presence of CaCO3 nanoparticles. The results of the tensile test can be explained by the presence of the two-counter balancing forces-the reinforcing effect of the CaCO3 nanoparticles and the decrease in spherulite size of the PP. Izod impact tests suggested that the incorporation of CaCO3 nanoparticles in PP has significantly increased its impact strength by approximately 300%. J-integral tests showed a dramatic 500% increase in the notched fracture toughness. Micrographs of scanning electron microscopy revealed the absence of spherulitic structure for the PP matrix. In addition, DSC results indicated the presence of a small amount of phase PP after the addition of the calcium carbonate nanoparticles. We believe that the large number of CaCO3 nanoparticles can act as stress concentration sites, which can promote cavitation at the particle-polymer boundaries during loading. The cavitation can release the plastic constraints and trigger mass plastic deformation of the matrix, leading to much improved fracture toughness.