The Izod impact strength of two kinds of ternary composites was investigated. One consisted of polypropylene (PP), the triblock copolymer polystyrene-block-poly(ethylene butene)-block-polystyrene (SEBS), and calcium carbonate (CaCO3) particles, and the other consisted of PP, carboxylated SEBS (C-SEBS), and CaCO3 particles. The mean size of the CaCO3 particles was about 160 nm. According to scanning electron microscopy observations, the composite with SEBS showed a morphology in which SEBS domains and CaCO3 particles were independently dispersed in the PP matrix. On the other hand, the composite with C-SEBS showed a morphology in which CaCO3 particles were encapsulated by C-SEBS; that is, a core-shell structure was formed. The Izod impact strength of the composite with SEBS was higher than that of the composite with C-SEBS and the PP/SEBS and PP/C-SEBS binary blends. According to observations of the fractured surface, the stress-whitened area was larger in the composite with SEBS than in the composite with C-SEBS and the PP/SEBS and PP/C-SEBS binary blends. The toughening mechanism of the composite, using nanometer-sized CaCO3 particles in combination with SEBS, was examined. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 919-927, 2009