Ternary composite of nano-CaCO3/ethylene-propylene-diene terpolymer (EPDM)/polypropylene (PP) with high content of nano-CaCO3 was prepared by two step compounding route, in which EPDM and nano-CaCO3 were mixed first, and then melt compounding with PP matrix. The influence of mixing time during the second Compounding on distribution of nano-CaCO3 particles and the impact strength of the ternary composite have been investigated. It was found that the Izod impact strength of composite decreased with increasing mixing time. The observation of transmission electron microscopy obviously showed that nano-CaCO3 particles transported from EPDM to PP matrix firstly and then from PP to the vicinity of EPDM dispersed phase with the increase of mixing time. This phenomenon can be well explained by the minimization of the dissipative energy and the Young's equation. The scanning electron microscope images show that lots of nano fibrils exist at tine interface between nano-CaCO3 agglomerates and matrix, which can dissipate lots of energy. The toughening mechanism has been interpreted in terms of three-stage-mechanism: stress concentration, void and shear band formation, and induced shear yielding. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:2485-2491, 2009