We fabricated a carbon nanotube based gas sensor for the detection of CO and NH3 mixed gas. Conventional photolithography technology was applied for the localization of sensing materials and the sensor fabrication. Single walled carbon nanotubes were dispersed in Sodium Dodecyl Sulfate(SDS) solution. The whole surface was covered by a photoresist except window areas between two electrodes. Carbon nanotubes were carefully dropped over the window areas and dried at 220°C over a hot plate. Then, the photoresist was removed using acetone. CO and NH3 mixed gas was delivered into a chamber at room temperature with nitrogen as a carrier gas. For comparison, each gas was applied to our sensor separately. Electrical resistance was measured for sensing performance.