Using Gaussian 09 software, we studied the toxicity of CO and ClCN on the surface of (8,0) single-walled carbon nanotubes (SWCNT) doped with transitional metal TM= (Fe, Ni, and Ru) using density functional theory (DFT) computation. There were other studies of the adsorption energy as well as equilibrium distances, gaps in energy, total energy, and density of states (DOS). According to the considerable energy gap fluctuation during gas molecule adsorption, TM=(Fe, Ru, and Ni)-doped gas senses adequately, according to the findings. Tests revealed softer and more reactive doped SWCNT systems had lower band gaps but higher ionization energies and electronegativity. Doping modified carbon nanotubes as a novel means of manipulating their electrical characteristics may be concluded, and the best sensor for both CO and ClCN is found to be CNT-Ni

Carbon nanotube , DFT, Senser,