Carbon nanotube gas sensor does not require heat for detecting gas species unlike present semiconductor type; hence, it is expected to be commercially available. The detecting mechanisms are explained by both charge transfer and Schottky barrier, but its detail such as contribution of polar molecule and charge variation are not clear. In this study, we examined as-grown vertically aligned single-walled and dispersed multi-walled carbon nanotube as sensors without any micro/nanofabrication for realizing simple gas sensor. Both film showed obvious response to the air and, their detecting principle is explained only by charge transfer between carbon nanotube and absorbed gas molecules. (C) 2015 Elsevier BY. All rights reserved.