A single-walled carbon nanotubes (SWNTs)-based gas sensor, fabricated by screen-printing method, was employed for ammonia (NH3) detection at room temperature. Upon exposure to NH3 gas, the resistance of the sensor positively increased. The sensor could detect NH3 at a concentration as low as 5 ppm at room temperature with a response time of 10 minutes. When the NH3 concentration increased, a saturation of the sensor sensitivity seemed to reach at 40ppm. Exceeding to 40ppm concentration, the sensitivity continued to rapidly increase. The effect of various operation conditions on the behavior of the SWNT-based gas sensor was reported. When the sensor was heated from room temperature up to 400°C, a transition from semiconducting into metallic behavior eventually took place. The sensor sensitivity was affected by operating temperature. The higher temperature the sensor operated at, the lower sensitivity it exhibited. By choosing a suitable regime of operation, in which the carrier gas flux, the heating temperature and heating time were appropriately controlled, the performance of the sensor was significantly improved in comparison with other sensors at the same type.