The gas sensitivity properties of carbon nanotubes were investigated. The carbon nanotubes in this study were fabricated by hot filament chemical vapor deposition using Ni/Fe catalyst. The sensor samples were treated by chemical etching using H2SO4/HNO3. The experimental results have shown that the electrical resistance of carbon nanotubes was sensitively changed on exposure to gaseous ambient containing ethanol and water vapor. It was found that the gas sensitivity of carbon nanotubes was greatly enhanced after chemical treatment. It was also shown that the response time of nanotube sensor was faster than that of usual sensors.