We investigate the electromechanical properties of ZnO grown on poly(ethylene terephthalate) (PET) and compare the results with those obtained from the counterparts on glass substrates. The elastic moduli of ZnO on PET (measured by a microstretcher) are very close to those of ZnO grown on glass substrates (measured by nanoindentation): The moduli decrease then increase as the O-2/Ar ratio increases. Films start to peel off from the plastic substrates when oxygen content reaches 50%. The in situ measurement of the ZnO resistance under uniaxial tensile stretching is influenced by the induced piezoelectric voltage, leading to a reduction in the electrical resistance for highly resistive ZnO films. The trend of the preferred orientation in relation to the oxygen content of the sputtering atmosphere is the same for ZnO films grown on either PET or glass substrates. The optical bandgap (E-g) of the ZnO films on both substrates exhibits crystal orientation dependence, varying from 3.18 eV with (002) preferred orientation to 3.25 eV, with a large number of (100) and (101) oriented crystals. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428416] All rights reserved.