A new type of CNTs based ammonia gas sensor was investigated. The array of carbon nanotubes was synthesized into the pore of anodized aluminum oxide (AAO) by thermal chemical vapor deposition method without metal catalyst. The anodized aluminum oxide was synthesized on the p-type Si(100) substrates by two step anodization process. After removing the carbon top layer, a thin Ag electrode was deposited on the surface of sample to get the opened end of the CNTs for gas sensor. The electrical and gas sensing properties of carbon nanotubes based sensor were characterized by I-V test after metal deposition on the top of carbon nanotubes. The device shows high sensitivity for detecting the NH3 gas at room temperature.