The important role of network relaxation in the voiding and fracture toughness of isotactic polypropylene (iPP) has been explored with video-aid tensile tests, two-dimensional small-angle X-ray scattering (2D-SAXS) measurements, and morphological observations. The results indicated that macroscopic volume dilatation related to voiding became lower during large deformation of iPP sample with rich gamma phase (denoted as gamma-iPP) cooled at 1 degrees C/min, compared with one with exclusive a form (referred to alpha-iPP) quenched in air. Furthermore, void propagation perpendicular to the tensile direction, demonstrated by Guinier approximation analysis of 2D-SAXS results, was suppressed to a large extent in such a gamma-iPP sample. Less network relaxation, resulted from its peculiar crystalline and amorphous phases, was responsible for lower volume dilatation and slower void propagation in the gamma-iPP sample. Meanwhile, less network relaxation and suppressed transverse void propagation contributed to higher toughness in the gamma-iPP sample. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 4092-4099, 2009