There are many traditional ways to improve sensitivity and selectivity of semiconductor gas sensors, such as metal ions adulteration[1,2] and surface modification[3,4]. In this paper 1.75 MeV electron beam was used to modify surface structure of tin dioxide gas sensors, and the gas sensing characteristics were studied. Results showed that the sensitivity and selectivity of SnO2 sensors were improved after the electron beam irradiation.The experiment was conducted on a GJ-2-II type electron accelerator (Shanghai Xianfeng Co. Ltd.), with an E-beam of 1.75MeV and 2mA. SnO2 sensor samples were irradiated for 10, 30, 60, 120 and 240s, and the doses delivered to the samples were 7, 21, 42, 84 and 168 kGy, respectively. Sensitivities to hydrogen and ethanol of the sensors were measured before and after the irradiation. As shown in Fig. 1, after the irradiation, sensitivities of the SnO2 sensors were improved greatly. For 100μg g-1 hydrogen with the 42kGy irradiated sensor, the sensitivity increased from 7.7 to 9.5. Further increase of the dose, however, would not lead to further increase of the sensitivity. For 100 μg g-1 ethanol, the same results were observed.The energetic electrons injecting into the SnO2 generate defects and active species like O- and O2-, which react with reductive gases such as hydrogen or ethanol, and these increase the sensitivity. As the irradiation dose increases, more active species are produced, hence the increased sensitivity. By computer simulation, it was found that higher irradiation dose would introduce more defects inside the SnO2 layer, leading to a better response to surface gas adsorption. But when the irradiation dose increased to a certain level, the effects on the structure tended to be saturated, as shown in Figure 1, the sensitivity became smooth when the irradiation dose increased to more than 100 kGy.