Carbon nanotubes (CNT) grown by chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD), and followed by annealing at $400{sim}500^{circ}C$ were investigated for gas sensing under 1.5ppm $NO_{2}$ concentration at an operating temperature of $200^{circ}C$. The electrical resistance of CNT sensor decreased with temperature, indicating a semiconductor type. The resistance of CNT sensor decreased with $NO_{2}$ adsorption. It was found that the sensitivity of sensor was affected by humidity and decreased under microwave irradiation for 3 minutes. The CNT sensor grown by PECVD had a higher sensitivity than that of CVD.