To clarify the thermal degradation mechanisms of uniaxially drawn poly(vinylidene fluoride) (PVDF) films, variations due to annealing in the polymeric structures of the films were investigated using the small-angle X-ray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The films were composed of lamellar crystals that were stacked perpendicular to the stretch direction. Although the crystallinity of the films decreased during annealing in the temperature range above the preannealing temperature, the lamellar structure was maintained even after the annealing process. There are two kinds of irreversible relaxation mechanisms during the annealing process of the films, including both a decrease in crystallinity within the lamellae and also thickening of the lamellae. A significant lamella thickening effect was observed when the films were annealed above similar to 100 degrees C. FTIR spectra suggested some disordered structures are developed during thickening of the lamellae. Furthermore, a long-range periodic structure was formed in the films that were annealed above the melting temperature of PVDF. The polymeric structures formed during the fabrication process (including high-order structures and c I disorders in molecular conformation) were clarified as having a significant influence on the annealing behavior of ferroelectric PVDF films. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2837-2841, 2009