In this study, the influence of phase separation on impact toughness of isotactic polypropylene (iPP)/poly(ethylene-co-octene) (PEOc) blends was investigated. For the typical toughened polymeric system, three iPP/PEOc compositions (80/20, 70/30, and 60/40) were selected. When the polymeric blends were annealed at 200 degrees C, the coarsening of phase domains was more prominent for the blend containing higher content of PEOc, and the scale of its morphological evolution was increased as well. The impact test showed that the impact strength variation trend as a function of annealing time was closely related to morphological evolution. It was believed that the sharpening of phase boundary and coarsening of phase domains were responsible for the depression of impact toughness, and the probable fracture mode alteration from shear banding to crazing and voiding. Structure evolution induced by phase separation showed an important effect on impact toughness, and it was also affected by the environmental conditions. Proper temperature was required to catch the tough-brittle transition induced by phase separation. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 445-453, 2011