The beta ->alpha phase transformation during annealing of beta-nucleated isotactic polypropylene (iPP) at 160 degrees C for 1 h has been confirmed by differential scanning calorimetry and X-ray diffraction measurements. The alpha-acrystals and related crystallinity in the annealed samples result in higher yield stress and volume dilatation, indicated by the true stress-strain and volume strain analysis, respectively. Meanwhile, higher volume dilatation can find its origin in the lower network modulus in the annealed samples. The increasing yield stress and volume strain, related to the crystalline phase (including crystal form and crystallinity), furthermore, should be responsible for the significant deterioration of the fracture toughness in the annealed samples determined by the essential work of fracture tests. (C) 2008 Wiley Periodicals, Inc.