Poly(vinylidene fluoride) (PVDF) thin films were subjected to tensile tests in plane strain over the temperature range from 65 to 115°C. At different elongation velocities using video real stress and strain, it was observed that, under such strain regimes, the neck-in starts perpendicular to the tensile axis and propagates towards the sample ends. The propagation kinetics was carefully recorded during the tensile tests. It was shown with simple hypotheses, that essential plasticity equations of the material can be determined via this method in terms of local effective stress (σeff) as a function of local effective strain (εeff) for every investigated temperature. The X-rays diffraction shows that the stretching of the sample of PVDF at a temperature lower than 72°C causes the transition from phase II to phase I.