A new kind of polypropylene (PP)/CaCO3 composites was prepared on a twin screw extruder with the nanoparticle content of 5 wt % and the 2500 mesh microparticle content of 15 wt %. The mechanical property of four different samples [pure PP (1), PP filled with 15 wt % microCaCO(3) particle composites (2), PP filled with 5 wt % nanoCaCO(3) particle composites (3) and PP filled with micro/nano-CaCO3 complex size particle composites (4)] was investigated through tensile tests, notched Izod impact tests and SEM. The results indicated that the sample 4 had the best mechanical property. The proofs of SEM showed that the high impact energy could lead to debonding and creating microcavitation between the nano-particle and polymer interface if the polymer was filled with the nanoparticles. This process could absorb a lot of mechanical failure energy, but too much mechanical failure energy would lead to the enlargement of microcavitation and the destruction of the composites in sample 3. In sample 4, the microparticle could be used to prevent the enlargement of microcavitation in the matrix polymer under the higher impact failure energy. In this article, the model of the impacting failure process of micro/nanoCaCO(3)/PP composites was established. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3560-3565, 2011