This paper described relative humidity (RH) sensing with ZnO based Film Bulk Acoustic-wave Resonator (FBAR). The resonant frequency of the FBAR decreased in a two-stage manner as the RH increased in the environment. For low RH (RH<50%), a frequency shift of 2.2 kHz per 1% RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film. While for high RH (RH>50%), a frequency shift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface. Ultraviolet (UV) light was applied to monitor its effects on the humidity sensing performance of the FBAR. UV can enhance the sensitivity at low RH (response increased to 3.4 kHz per 1% RH change), while degrade the sensitivity at high RH (response decreased to 5.7 kHz per 1% RH change). This study has proven the feasibility of measuring relative humidity using ZnO film based FBAR.