In the study, a MEMS-based gas sensor is presented, which consists of a sensing thin film deposited by RF sputtering and annealed at 375°C. The structure and surface pattern of the thin film are analyzed by XRD and SEM. The sensor consists of a substrate, Pt interdigitated electrodes and an SnO2 sensing layer. As concentration of oxygen changes, a change in the electrical conductivity of the SnO2 film is caused. The experimental results show that the measured resistance increases as the concentration of oxygen increases at a working temperature of 300°C. A good oxygen sensing performance is presented in the study.