Piezoelectric nanogenerators (PNGs) based on ZnO nanowires embedded in a polymer matrix have shown great promise in converting ambient mechanical energy into electrical energy, positioning them as candidates for autonomous sensor applications. Here, we fabricate vertically integrated ZnO NW/parylene-C composite-based PNGs using a capacitive configuration. By carefully controlling the thickness of the parylene-C top layer over ZnO nanowire arrays, four PNGs with parylene-C top layer thicknesses ranging from 1.1 to 3.2 µm were successfully fabricated. Raman spectroscopy suggests that the parylene-C does not affect the crystallographic properties of ZnO nanowires when coated. In addition, electrical impedance measurements reveal that increasing the parylene-C top layer thickness decreases the PNG capacitance, leading to higher internal impedance. The performance of these PNGs is assessed through piezoelectric characterizations across a range of load resistances, from 50 kΩ to 122 MΩ, under vertical compression forces of 1 N applied at 0.2 Hz. These tests have identified an optimal parylene-C top layer thickness of around 2 µm, resulting in an instantaneous power density of 1.8 µW/ cm 3 generated by the PNG. These findings highlight promising pathways for enhancing the efficiency and performance of PNGs.