This work mainly focuses on the behavior of gas sensors base on carbon nanotube (CNT) bundles in various circumstances. The extraordinary properties of the CNT, the commonly used methods for CNT synthesis and the huge potential applications of the CNT, including gas detection, were reported. The effects of operating and pretreating parameters, such as heating, increasing in fluxing gas, pre- annealing and acidic stirring at various conditions, were thoroughly studied. Some abnormal behaviors of the CNT bundles, that took place during the investigation, have been discovered and discussed in the work. From the experimental results, the optimal condition for operating and pretreating of the CNT-based gas sensors was suggested.