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Thick-film gas sensors are successfully fabricated using the nanostructure tin-oxide powder. In order to suppress the coarsening of the nanostructure tin-oxide particles during the adding process, the low-temperature catalyst adding (LTCA) method is proposed in this paper. LTCA is an adding method of noble Pd catalyst onto the nanostructure tin-oxide particles under the lower temperature below 300°C by excluding chloride. It turned out that the adding without particle coarsening is successfully carried out by means of LTCA. Applying LTCA to the fabrication of the thick film using nanostructure tin-oxide powder having a size smaller than 5 nm leads to an excellent performance with respect to the methane gas sensing. After aging at 400°C, a good sensitivity (R3500/R1000) of 0.66 is obtained for the sensor doped with 5 wt% of Pd catalyst. Also, the sensitivity of the sensor is so stable that the deviation of the electrical resistance is within 3% after 400 h of aging.