A solid-state gas sensor with the structure of Pt micro grid (sensing electrode)/ LaF₃ (solid electrolyte)/ Sn, SnF₂ (reference electrode) was developed for detecting oxygen at room temperature. In the oxygen sensor, Pt micro grid with the thickness of 60nm was fabricated on one side of the solid electrolyte (LaF₃) wafer by lift-off process, which was used as a novel structure of sensing electrode for the purpose of changing the diffusion process of gas species and investigating the relationship between the sensing electrode dimensions and properties of the sensor. In order to form SnF₂ layer on the other side of the LaF₃ wafer, Sn film about 2μm thick was deposited on LaF₃ wafer using sputtering technique first. Then the sensor cell was electrolysed at 0.2μA for 6 min in air at room temperature. The sensing mechanism was investigated in detail and it was found that the electromotive force(EMF) of the sensor depends on the partial pressure of O₂. The sensing properties of the sensor were also tested.