Density functional theory (DFT) studies were performed to evaluate the ability of the pristine (8, 0) and (10, 0) BN nanotubes as sensor for cyanogen chloride (CNCl). The adsorption of CNCl reacting with BN nanotubes are studied at B3LYP 6-31G* level of theory. Compared with carbon nanotube, (8, 0) BN nanotubes can strongly adsorb the CNCl molecule (about −0.22 eV) with −9.31% change in the energy gap. Our results reveal that the length and diameter of tube have not significant effect upon the adsorption process. In contrast with Ga-doped BN nanotube, the Al-doped BN nanotube has a significant change in the energy gap (about −21.44%) with the adsorption energy of −1.01 eV. Therefore, Al doping BN nanotubes can be utilized as a sensor for toxic CNCl.