Previous work showed that annealing induced the great improvement of fracture resistance of beta-iPP, relating to the decreased number of chain segments in the amorphous region. To further prove the rationality of this observation, in this work, the ethylene-octene copolymer (POE) toughened isotactic polypropylene (iPP) blends with or without beta-phase nucleating agent (beta-NA) were adopted and the changes of microstructure and fracture resistance during the annealing process were further investigated comparatively. The results showed that, whether for the a-phase crystalline structure (non-nucleated) or for the beta-phase crystalline structure (beta-NA nucleated) in iPP matrix, annealing can induce the dramatic improvement of fracture resistance at a certain annealing temperature (120-140 degrees C for beta-NA nucleated blends whereas 120-150 degrees C for non-nucleated blends). Especially, non-nucleated blends exhibit more apparent variations in fracture resistance compared with beta-NA nucleated blends during the annealing process. The phase morphology of elastomer, supermolecular structure of matrix, the crystalline structure including the degree of crystallinity and the relative content of beta-phase, and the relaxation of chain segments were investigated to explore the toughening mechanism of the samples after being annealed. It was proposed that, even if the content of elastomer is very few, the excellent fracture resistance can be easily achieved through adjusting the numbers of chain segments in the amorphous phase by annealing. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2108-2120, 2010