Direct piezoelectric g 31 voltage coefficient was measured in situ as a function of applied tensile stress for films of polyvinylidene fluoride (PVDF). Measurements were performed under quasi-static conditions with applied strain rates of 0.5-1.5 mm/min for strains up to 12%. Open-circuit voltage was measured with a contact-less electrostatic voltmeter. Obtained results show a strong dependence of the g 31 coefficient of mono-oriented PVDF films on the applied stress, with a maximum value of the coefficient in the transition region between elastic and plastic deformation zones. The effect of sample geometry on the apparent g 31 coefficient is shown and discussed. The anisotropy of the piezoelectric effect is studied by means of g 31 and g 32 measurements. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43248.