The structural evolution (not only intra-fibrillar but also inter-fibrillar structures) during extension of poly (vinyl alcohol) (PVA) in water at different temperatures are investigated by in-situ small-and wide-angle X-ray scattering (SAXS and WAXS). The stress-strain curves can be defined into four zones with the onset and the end of stress plateau, and the strain hardening point as the zone boundaries. WAXS results show that stretch induces melting of PVA crystal from zones I to III, while SAXS measurements reveal stretch-induced transformation from initial lamella to nanofibrils starts at the onset of zone II, which proceeds via a melting-reconstruction process instead of direct transformation. In zone III, the nanofibrils increase in content and pack periodically with an average inter-fibrillar spacing of about 14-18 nm. Increasing temperature leads to larger inter-fibrillar spacing and more perfect packing of nanofibrils, which may benefit the production of polarizer with homogeneously structural and optical properties. (C) 2018 Elsevier Ltd. All rights reserved.