Abstract: By using Density Functional Theory (DFT), study the electron properties of the fullerene molecules adsorbed with gas (COCl₂). The gas with the molecule was placed with two sites at the first position (S1) vertically on one of the carbon atom for C60 molecule, vertically on one of the chlorine atom for (C59Cl and C58Cl₂) molecules and vertically on one of the bromine atom for (C59Br and C58Br₂) molecules. The second position (S2) is at the center of a ring of six carbon atoms of pristine fullerene (C60), center of a ring of six atom of (C59Cl and C58Cl₂) molecules and center of a ring of six atom of (C59Br and C58Br₂) molecules using the functional (B3LYP) and the basis set 6-31G (d, p). The results show that, the lowest adsorption energy obtained by gas adsorption with the molecule (C59Br) at the sites (S1) and the value of (-0.627 eV) and (C58Cl₂) at the sites (S2) and the value of (-0.654 eV) through this result can be considered this case gas sensor to detect gas (COCl₂). The rest of the cases is weak sensors and can be used as a sensor assistant.