Naphthenic acids are naturally occurring compounds that constitute part of the petroleum acids. They are corrosive species, and are toxic to a variety of organisms. Therefore, the removal of them is regarded as one of the most important issues. In this study, density functional theory approach has been devoted to investigate the adsorption of some naphthenic acids (e.g., benzoic acid, cyclohexan carboxylic acid, cyclohexan propionic acid) on magnesium oxide. The effect of aromaticity of the ring and the hydrocarbon chain of the naphthenic acids has been explored. Interaction energies, dipole moment, polarizability and energies of frontier orbitals have been calculated.