The effect of operating temperature on characteristics of single-walled carbon nanotubes (SWNT) based gas sensor was investigated. SWNT-based sensor was fabricated from SWNT powder (Iljin Nanotech, Korea) by screen-printing method. SANT powder (30 mg, AP grade) was dispersed into 0.78 gram alpha-terpineol (Aldrich) by ultrasonic vibration for I hour then stirred manually for I hour to increase adhesion. From this condensed solution, a thick film of SWNT was printed onto alumina substrates. The film then was sintered at 300 degrees C for 2 hours to remove residual impurities. Upon exposure to some gases such as nitrogen, ammonia or nitric oxide, resistance of the sensor dramatically changes due to gas adsorption. In our experiments, SWNT-based sensor was employed to detect NH3 gas in N-2 ambience. After saturated of N-2, the sensor exposes to NH3 with various concentrations (from 5 ppm to 100 ppm, diluted by N-2 as carrier gas). This sensor exhibits a fast response, high sensitivity but slow recovery at room temperature. By heating at high temperature and increasing the flow-rate of carrier gas, NH3 gas desorbs easily and recovery of the sensor improved. The heating also influenced the characteristics of sensors such as response and reproducibility. Other special changes in electric property of SWNT-based sensor caused by heating are also discussed.