Influence of alpha- and beta-nucleation on brittle-ductile transition temperature (BDTT) of impact-resistant polypropylene copolymers (IPCs) and their phase morphologies were comparatively investigated. Impact test showed that the BDTT of beta-nucleated IPC (beta-IPC) is similar to 24 degrees C lower than that of a-nucleated one (alpha-IPC). Structural characterizations including atomic force and scanning electron microscopic observations, small angle X-ray scattering examination, and dynamical mechanical analysis revealed that dispersion of the ethylene-propylene random copolymer-rich (EPR-rich) phase was finer in beta-IPC in comparison with that in alpha-IPC. For the reason of looser lamellar arrangement, the portion of EPR-rich components included in the interlamellar region of beta-IPC was higher than those of alpha-IPC, which led to improved mobility for the amorphous polypropylene chains. It was proposed that the finer distribution of EPR-rich phase, which might result from faster growth rate of the beta-crystal and looser lamellar arrangement of beta-spherulite, should be responsible for the improved impact-resistance and lower BDTT in beta-IPC samples. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1784-1792, 2012