Tin oxide based hydrocarbon gas sensors were prepared by using various methods and their sensing characteristics and the effects of high temperature treatment and additives on the sensitivities were tested. Although the choice of the effective noble metal depended on the detecting gas, it was found that the supported noble metal catalyst was the most effective additive to increase the sensitivity of the sensor. Especially well dispersed 5wt% of Pt and Pd supported alumina catalysts were the most effective for the butane and methane sensor, respectively. The increase in sensitivity was due to the increase in dispersion of the noble metal used and the sensors with supported catalyst were found to be very stable up to the temperature of 600℃. When the sensor was calcined at 800℃, the thermal instability of the sensor increased with metal dispersion regardless of preparation methods, resulting in the decrease in sensitivity. The thermal instability at a high temperature could be explained by the dispersion of the Pd metals rather than the sintering of tin oxide or supported metal catalyst itself or additives