This paper presents the loss in piezoelectric performance of piezopolymer film sensors like PVDF and P(VDF-TrFE) and P(VDF-TrFE)/ZnO nanocomposite thin films due to thermal exposure. Changes in the material structure through XRD diffraction analysis reveal the high stability of P(VDF-TrFE) and P(VDF-TrFE)/ZnO nanocomposites films after subjecting to thermal exposure above 120 °C. Loss in the crystalline β-phase of PVDF film was observed after thermal exposure above 80 °C accompanied by shrinkage. Loss of crystalline β-phase is lesser in constrained films as compared to free films. Next, the piezoelectric coefficient d33 measured using dynamic force method suggests highest stability of PVDF film at higher temperature as compared to P(VDF-TrFE) and P(VDF-TrFE)/ZnO nanocomposite thin film. In addition, the constrained PVDF film shows better stability of piezoelectric coefficient d33 than the unconstrained PVDF films. Using the guided wave the piezoelectric coefficient d31 of these films was measured. Among these films, PVDF films show excellent stability in the piezoelectric coefficient d31 when subjected to thermal exposure. P(VDF-TrFE) and P(VDF-TrFE)/ZnO thin films retain piezoelectricity up to 90 °C, whereas PVDF films retain piezoelectricity up to 120 °C.