In this study, the effects of alpha- and beta-nucleating agents (alpha-NA and beta-NA) on the toughening behavior of impact polypropylene copolymer were ascertained with respect to three test temperatures (23, 0 and -15 degrees C). The addition of alpha-NA impacted the toughness slightly for all test temperatures. However, the tendency of impact strength vs. beta-NA content at 0 degrees C significantly differs from that of the other two temperatures. Importantly, a close correlation between toughness at 0 degrees C and chain mobility of the amorphous portion has been well revealed for the first time by achieving a linearly fitting between impact strength at 0 degrees C and beta-relaxation peak intensity in DMA spectrum. A comparative investigation between the situations containing alpha-NA or beta-NA offers new insights into the physical origin of the toughening behavior for a multiphase multicomponent polyolefin system. The chain mobility of matrix amorphous portion plays a dominant role on toughening. (C) 2012 Elsevier Ltd. All rights reserved.