In this paper, we report on the flexible thin film piezoelectric nanogenerators based on two-dimensional ZnO nanoflakes (NFs) directly deposited onto flexible polyethylene terephthalate (PET) using a simple sonochemical reaction in aqueous solution at room temperature. Our sonochemical synthesis method is a rapid, highly stable, low-cost, and reproducible method, which can be performed at ambient conditions. These advantages of the sonochemical method allow the synthesis of many different ZnO nanostructures. The structural investigations using scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) indicated that the ZnO NFs grew with high level of crystallinity and without any thermal damage on the substrates. The fabrication of these device provides a promising solution for developing flexible and self-powered electronic devices particularly wearable and implantable sensors. This ZnO-NFs based nanogenerator provides 62 mV of potential and significant reproducibility having with lower p-value (0.0212).