A new toughening agent, polypropylene/poly(ethylene-co-propylene) in-reactor alloy (EP-P), has been adopted to modify isotactic polypropylene (PP) in present study. Systematic investigation has been performed on the inter-compositional interaction, crystalline structure, and phase morphology of a series of PP/EP-P blends. It has been found that the PP component from EP-P is thoroughly miscible with neat PP and they together serve as the matrix of the PP/EP-P blends, while the ethylene-propylene random copolymers (EPR) act as the dispersed phase. The ethylene-propylene segmented copolymers (EPS), behaving as the compatibilizer between the EPR dispersed phase and PP matrix, strengthen mutual incorporation and effective diffusion of the amorphous PP segments and the EPR molecules. Based on the in-depth understanding of the crystalline structure and phase morphology, the correlation between morphological structure and mechanical properties has been established. The excellent impact toughness of PP/EP-P blends with higher EP-P content is mainly attributed to the small PP crystallites scattered in the blends and the well dispersed EP copolymer domains in PP matrix. (C) 2009 Elsevier Ltd. All rights reserved.