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In this paper, the tin oxide thin film gas sensors were successfully manufactured under a certain process condition and the humidity sensing properties of the devices varying with the operating voltage were tested. To begin with, a model of the gas sensor was built and simulated to determine the steady-state temperature distributions of the whole device and the gas sensing film using ANSYSTM. Then X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the tin oxide film fabricated under the specific process condition, showing that it has a high oxygen content. At last the humidity sensing characteristics of the devices at different operating voltages were tested. A conclusion can be drawn from the experimental results that the humidity sensitivity of the gas sensors can be enhanced from 1.429E-2 to 4.651E-2 when the operating voltage is increased from 3.000 V to 4.000 V and can be reduced from 4.651E-2 to 4.077E-3 when the operating voltage is increased from 4.000 V to 5.000 V.