Abstarct: Zinc oxide is one of the most important II-VI group semiconductors, which has specific properties such as broad band gap of 3.37 eV, make it applicable in high frequency electronic components, high power lasers, high-speed field effect transistors and etc. Due to its high biocompatibility, this material can have new applications in the optoelectronics, sensors, and medicine. mextal oxides baxsed semiconductor gas sensors turn to be significant and broadly used in the environmental protection, disease diagnosis ant etc. In this study, the physical properties of the adsorption of the toxic gases of HF, C2N2, HCN, SO2, CNCl on pure ZnO single wall nanotubes were studied by using the VDW-DFT by the SIESTA computational code. The calculation have been made on (7,0) zigzag and (4,4) armchair single walled ZnO nanotubes. First, the electronic properties and band structure of the nanotubes were investigated and then the gas molecules were adsorbed on the nanotubes surfaces in both horizontal and vertical states, and the stable adsorption site was calculated. The pure zigzag and armchair nanotubes before adsorption, having a band gap of 2.16 and 2.22 eV respectively. The most stable adsorption position for molecules is obtained on the center of hexagonal and in the distance of 3 angstroms. Also the adsorption distance, the value of band gap and the charge transfer between the molecule and nanotube are calculated, and then the band structure and the total and partial density of states for the optimized structures is calculated. It was observed that the absorption of gas molecules with the surface of the nanotubes in horizontal modes is chemisorption while in vertical modes is physisorption. Moreover in chemisorption states the adsorption distance decreases and the amount of charge transfer between the gas molecule and the nanotube increases. In the chemisorption, the adsorption energy is obtained lower than -1 eV. The results indicate that zinc oxide nanotubes can be a good adsorbent for detecting of HCN and CNCl gases.