8 mol %-yttria-stabilized zirconia (YSZ) thin films as an oxygen ion conductor were deposited by radio frequency magnetron sputtering, and the oxygen gas sensing properties of YSZ were investigated using the structure of SiO2 substrate/Ni-NiO/Pt/YSZ/Pt. X-ray diffractometry was employed to study the structure of YSZ and Ni-NiO films, and energy dispersion X-ray was used to investigate the composition of Ni-NiO thin films. The gas-sensing test was carried out for a SiO2/Ni-NiO/Pt/YSZ/Pt film structure exposed to oxygen-controlled environments. The steady-state electromotive force (EMF) values were measured as a function of oxygen partial pressure (P-O2; 1.013 X 10(3) to 1.013 X 10(5) Pa) and operating temperature (573 to 973 K). The fabricated gas sensor calls showed good oxygen sensing properties at the temperature range from 673 to 773 K. However, the sensors were unstable at the operational temperatures above 873 K possibly due to the enhanced interdiffusion of the materials in the multilayer. Also the operation of the sensor at temperatures below 573 K was not good because the temperature was not sufficient to cause ionic conduction in the cell. At the optimum temperature range, the experimental EMF values measured as a function of oxygen partial pressure were close to the theoretically calculated EMF values.