The mixed-conductive ceramic oxide Gd0.2Ce0.8O2-delta (GCO) particles with 40-50 nm were synthesized by using a combined citrate and EDTA complexation method. The material was characterized with powder X-ray diffraction, transmission electron microscopy, energy dispersive spectrometry, and X-ray photoelectron spectroscopy. A humidity sensor was fabricated by screening GCO onto a ceramic substrate with a pair of interdigitated electrodes. The sensor shows a linear relationship between logarithm impedance and relative humidity in the range of 33-98% when the measurement frequency range is 20 Hz-1 kHz. Typical response and recovery times of the sensor are 40 and 210 s, respectively, indicating that desorption rate of water molecule inside the GCO material is slower than the adsorption rate. The humidity sensing mechanism was discussed.