Large amount of work has been reported on the annealing of polypropylene (PP) and the related changes in mechanical properties. However, the structure-property correlations and the physical origin of annealing induced microstructural evolution are still not very clear. In this work, taking beta-form PP (beta-PP) as example, the microstructural changes induced by annealing were investigated from macromolecular to crystalline lamellae level with Fourier transform infrared (FTIR) spectroscopy, conventional differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). Besides mobile amorphous fraction (MAF), the role of rigid amorphous fraction (RAF) in toughening PP is particularly taken into consideration. It is shown that annealing increases the chain mobility in MAF and decreases it in RAF. Such an effect is believed to be mainly associated with the formation of looser MAF and more RAF by the microstructural re-arrangement involving conformational ordering of partial amorphous chain segments and a significant interlamellae thickening. A thorough analysis of structure-property relationship through observing plastic deformation behaviors by scanning electron microscope (SEM) and estimating stress transmission between crystalline and amorphous phases, suggests that both MAF and RAF play important role on toughening beta-PP. They can promote the initiation of microvoids effectively upon deformation by reducing the stress transmission. As a result, large-scale plastic deformation is triggered. This work is important and provides a new insight into the mechanisms of microstructural evolution and subsequent improvement in impact toughness during annealing. (C) 2011 Elsevier Ltd. All rights reserved.