Abstract Density functional theory was used to investigate the adsorption of imidazole (IMD), benzotriazole (BTAH) and 2-mercaptobenzoxazole (MBO) on the grain boundary (GB) of Cu (100) surface. Results show that molecules prefer to adsorb on the grain boundary with the larger distortion degree and more adsorption sites. The main interaction of molecules and grain boundary is chemisorption through C, N, S Cu bonds in both perpendicular and parallel modes. Hydrogen bonds and the lateral dipole-dipole interactions are also found. Based on the adsorption behaviour, the coverage effect and charge redistribution might be two possible inhibition mechanisms of inhibitor molecules.