Nano-CaCO3/homo-PP composites were prepared by melt-blending using twin-screw extruder. The results show that not only the impact property but also the bending modulus of the system have been evidently increased by adding nano-CaCO3. The nano-CaCO3 particles have been dispersed in the matrix in the nanometer scale which was investigated by means of transmission electron microscopy (TEM). The toughening mechanism of nano-CaCO3, investigated by means of dynamical mechanical thermal analysis (DMTA) and scanning electron microscopy (SEM), lies on that the nano-CaCO3 particles take an action of initiating and terminating crazing (silver streak), which can absorb more impact energy than the neat PP. At the same time, the nano-CaCO3 particles, as the nuclear, decrease the crystal size of PP, the results of which were investigated by means of polarized optical microscope (POM). (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 911-916, 2010