Abstract The nano-structured thin NiO films were prepared by r.f. magnetron sputtering of pure nickel on an yttria-stabilized zirconia (YSZ) substrate, followed by oxidation and final sintering at different temperatures of 1000–1200 °C (1 h). The scanning electron microscope (SEM) observation showed that the thickness as well as the morphology of the thin NiO films depended greatly on the sputtering time and the sintering temperature. The thin NiO films formed having thicknesses of 30–180 nm were examined as a sensing electrode (SE) for the mixed potential-type planar YSZ-based NO 2 sensor operating at 600–800 °C. The sensitivity of the sensors using the thin NiO-SEs was found to be linear on a logarithm of NO 2 concentration in the range of 50–400 ppm and 120-nm thick NiO-SE sintered at 1100 °C gave the highest NO 2 sensitivity as well as very fast response/recovery at operating temperatures below 800 °C. Also, the fabricated NO 2 sensor using nano-thick NiO-SE showed much higher NO 2 sensitivity under exposing to the wet sample gas containing 5 vol.% water vapor than that under dry condition. This seems to be attributed to the high catalytic activity to the electrochemical reaction of NO 2 at the interface between SE and YSZ. In addition, the sensor using the thin NiO-SE exhibited always higher NO 2 sensitivity than that attached with thick NiO-SE fabricated by screen-printing technique.