In this paper, gas sensors employing nanoscale tin oxide films of high oxygen vacancies were successfully made and tested in order to determine which factors affected the gas sensitivity of them. Firstly electrothermal simulations were carried out to predict the steady-state temperature distributions of the tin oxide thin film and the overall gas sensor using ANSYS™. Then the composition of the tin oxide calcined twice in O₂ was analyzed by X-ray photoelectron spectroscopy (XPS). The result of XPS showed that the tin oxide under this condition had high oxygen vacancies. Finally, the ethanol sensing and humidity sensing properties of the gas sensors at various operating voltages and calcined in different conditions were tested. The experimental results showed that high gas sensitivity of the gas sensors could be achieved by changing the internal and external factors.