This article investigates a thin-film bulk acoustic-wave resonator (TFBAR) based on Li-doped ZnO (LZO) film. The LZO thin-film is deposited by radio frequency (RF) magnetron sputtering system and post-treated with ultraviolet (UV)-ozone illumination. The microstructural and chemical evolutions via various illumination times of the predominantly c-axis orientation LZO films are investigated. The largest piezoelectric coefficient (12.66 pC/N) of the LZO film is obtained after 90 min UV-ozone illumination, which can be ascribed to better crystallization and fewer oxygen-related defects. Furthermore, the fabricated TFBAR exhibits a high quality factor (Q = 1358) at ∼500 MHz and a good sensitivity under several relative humidity levels (30∼90% at room temperature in a standard atmosphere). The experimental results verify the LZO-based TFBAR is a candidate for humidity sensor applications.