Carbon nanotube (CNT) is receiving a lot of attention as gas sensor material because it has the properties suitable for gas sensor such as its large surface area, chemical stability and good sensitivity at room temperature. At present two detection mechanism were proposed: Schottky barrier between electrodes and CNT and charge transfer between gas molecules and CNT. Regardless of many researches, sensing mechanism of multi-walled CNT gas sensor has not been investigated unlike that of SWNCT. In this study, we focused on simple CNT gas sensor which consisted of as-grown Multi-walled CNT and tried to elucidate the detection mechanism using transmission line method.