Multi-layered zirconia (ZrO2 thin film on the Pt/Ti/SiO2/Si substrate has been prepared as humidity sensing material by sol–gel method. Annealing temperature was 450 C. A TiO2 thin film was added in between ZrO2 film and the substrate for improving the adhesion. The films were characterized by using X-ray diffraction (XRD), atomic force microscope (AFM) and Fourier-transform infrared (FT-IR) spectra. The impedance of the humidity sensor changes about four orders of magnitude within the relative humidity (RH) range of 11–98%, indicating a good sensitivity. The response time of the
sensor was about 5 s. The humidity sensing mechanism was discussed by means of the complex impedance curves and an equivalent circuit of the sensor.