Abstarct: Gas sensors are called components that can convert the concentration of a particular gas into an electrical signal. Zinc oxide is one of the most important semiconductors that has always shown good gaseous sensitivity. Zinc oxide is a semiconductor with a broad-band gap of 3.37 electron volts. In this study, the physical properties of the adsorption of the toxic gases of HCl-C2N2-HCN-H2S-CNCl on pure ZnO nanosheets were studied using the VDW- DFT by the SIESTA computational code. The calculation have been made on (3,3) armchair and (5,0) zigzag ZnO nanosheets. The gas molecules are adsorbed on the nanosheet surface in both horizontal and vertical modes at an optimized distance of 2.5 angstroms and the absorption energies are calculated after optimization. The adsorption distance, the value of band gap and the charge transfer between the molecule and nanosheet 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 gas molecules in the horizontal adsorption modes have more negative energy with chemical adsorption. Moreover in chemisorption states the adsorption distance decreases and the amount of charge transfer between the gas molecule and the nanosheet increases. By calculating the electronic band structure, it is observed that the band gap in the chemical adsorption states is zero and the structures show mextallic property. Furthermore the most negative energy is obtained for HCN molecule, suggesting that the (5,0) zigzag and (3,3) armchair ZnO nanosheets could be a good sensor for the HCN gas.