Abstract: This paper describes an application of quartz crystal microbalances（QCM） coated with nanocrystalline ZnO used as relative humidity sensors.The technique to prepare the nanocrystalline ZnO and the system to proceed the relative humidity testing are presented.Sensing mechanism is examined by measuring the resonance frequency shift of QCM which due to the additional mass loading.The results showed that the sensors developed have high humidity sensitivity,good reproducibility.