Abstarct: The gas sensors have attracted much attention in the world of science because of its wide use in the factory, industry, environment, etc. Boron nitride nanotubes (BNNTs) are wide gap semiconductor. Their electronic properties are independent of diameter, chirality, the number of the tube wall and high environmental temperature. They are promising for applications in nanoelectronics devices and components such as sensors because of excellent properties. In this research, using the SIESTA code baxsed on density functional theory with the generalized gradient approximation (GGA) the adsorption of carbon monoxide, hydrogen fluoride, cyanogen chloride, phosgene and hydrogen sulfide, on zigzag (6,0) and armchair (3,3) boron nitride single walled nanotubes have investigated for both pure nanotubes and doped with Al and Ga. For this purpose, the electronic and structural properties of nanotubes have investigated. Then gas molecules were absorbed on nanotubes surfaces and eventually have achieved stable absorption site. The most stable position for the adsorption of all molecules is found in the center of hexagonal in pure BNNTs. To evaluate the sensitivity and the effect of absorption on nanotubes, the electronic properties of nanotubes and charge transfer have been studied after absorption of gas molecules. The electronic structures show that pure zigzag and armchair nanotubes before adsorption, having a band gap of 2.677 and 4.224 respectively, and after absorption, the band gap variations is very small. The results revealed the very low effect of gas molecules absorption on the electrical properties of pure nanotubes, which be indicative of physical adsorption. The results revealed that (6,0) BN nanotube has better sensitivity. In order to achieve better results, (6,0) zigzag nanotube doped with Al and Ga atoms and the adsorption energy have investigated, the most stable equilibrium site, transferred charge and electronic properties have calculated.The most stable position for the adsorption of all molecules is found on the impurity atom. The adsorption energy in the stable positions has reduced and was in the chemical absorption range. The results show that the electronic properties of doped nanotubes have improved and conductivity of nanotubes is increased. Also the results revealed that the boron nitride nanotubes doped with Al is a good sensor for CNCl molecules.