Previous work showed that there was a synergistic effect of nucleating agent (NA) and elastomer in improving the fracture resistance of isotactic polypropylene (PP), relating to the formation of large amounts of beta-PP (beta-NA nucleated system) or the decrease of the spherulites diameters of alpha-PP (alpha-NA nucleated system). To find the direct relation between the synergistic efficiency of NA/elastomer and the microstructures of the materials, in this work, the ethylene-propylene-diene terpolymer (EPDM) modified PP blends with compounded NAs (beta/alpha) were adopted and the changes of the microstructure and mechanical properties were investigated comparatively. The results showed that, with the adjustment of the mass fraction of compounded NAs, the microstructures of PP matrix including supermolecular structure and the relative fraction of beta-PP (K-beta) change accordingly. Specifically, the Kb of beta-PP was successfully adjusted in the wide range of 0-78.9%. Consequently, the stiffness and the fracture resistance of the PP/EPDM blends were easily controlled in different degrees. It is believed that this work could provide a guide map for the design and preparation of certain polymer blends satisfying certain requirement. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 898-906, 2011