The effect of the addition of copper nanoparticles on the transformation from alpha to beta phase found in PVDF by stretching and thermal treatments was studied in this paper. These PVDF crystalline changes were in situ observed, evaluating simultaneously the uniaxial deformation process and the structural evolvement by 2D X-ray diffraction with synchrotron radiation. The results showed that the presence of copper nanoparticles facilitated the development of the beta-phase instead of the alpha-polymorph in PVDF. Thus, less deformation is needed in the nanocomposite compared with that required in the pristine PVDF to achieve the same amount of that beta polymorph, and a higher proportion of this beta phase is obtained at the same level of deformation. This increase in the content of the beta phase was also corroborated by FTIR spectroscopy. (C) 2015 Elsevier B.V. All rights reserved.