To improve the toughness of isotactic polypropylene (iPP), by blending modification or special processing techniques, is an ongoing pursuit in the polymer community. In this study, the toughness of injection-molded iPP bars has been enhanced by about four times upon annealing at 140 degrees C, in parallel with slight increasing of stiffness. Through various structural characterizations it has been confirmed that the remarkable toughening effect in the annealed iPP bars mostly results from the enhanced molecular mobility in the amorphous phase due to lamellar perfection, rather than from the change of molecular orientation, crystal modification, crystallinity, and so on. Moreover, it is deduced that the gain of mobile amorphous fraction in the hierarchical iPP bars upon annealing follows the order of skin > intermediate > core, same to that contributing to the toughness enhancement. POLYM. ENG. SCI., 52:893-900, 2011. (C) 2011 Society of Plastics Engineers