8mol%-yttria-stabilized zirconia(YSZ) thin films as oxygen ion conductor were deposited by rf-magnetron sputtering, and the oxygen gas sensors with the structure of $SiO_2$ substrate/Ni-NiO mixed reference layer/Pt/YSZ/Pt were fabricated and their oxygen sensing properties were investigated. The steady-state electro-motive force (EMF) values were measured as a function of oxygen partial pressure ($PO_2;form 1.013 imes10^3 extrm{Pa ;to}; 1.013 imes10^5$Pa) and operating temperature ($300^{circ}C$ to $700^{circ}C$). The fabricated YSZ oxygen sensor showed the best oxygen sensing properties at 50$0^{circ}C$. However, oxygen sensing properties were very low at the temperature lower than 30$0^{circ}C$ due to the lack of oxygen ion mobility and at the temperature higher than $700^{circ}C$ due 새 intermixing of materials between the layers. Especially, the YSZ sensor operating at $500^{circ}C$ and oxygen partial pressure above $1.565 imes10^4$Pa showed the oxygen sensing properties close to the values predicted by ideal Nernst equation.