We fabricated the gas sensor using carbon nanotubes (CNTs) and investigated the detection properties for NH/sub 3/ gas. The sensing material of gas sensor is employed multi-wall carbon nanotubes (MWNTs). The CNT gas sensor structure is realized with technologies of micro-electro mechanical systems (MEMS). The nanotubes gas sensor is operated at room temperature. And the electrical resistance of the carbon nanotubes is measured upon exposure to NH/sub 3/ gases. Also, recovery time of CNT gas sensor is measured according to temperature.