Thin SrTiO3 / La0.7 Sr0.3 MnO3 (STO/LSMO) films of 30 nm thickness were epitaxially grown on a (001) Pb (Mg1/3 Nb2/3) O3 -28 mol % PbTiO3 piezoelectric single-crystalline plate and investigated by means of wide-angle x-ray scattering in situ under the influence of a direct current electric field with strength E up to ±18 kV/cm. The strain s in the films could by tuned dynamically due to a dominant converse piezoelectric effect of the substrate. The coefficient of coupling between electric field-induced strain alteration of the substrate and the buffer film (LSMO) is approximately 0.75, whereas for the substrate and top films (STO) it is about 0.35 related to a normalized strain alteration of the substrate of 1.