Quantum chemical calculations using density functional theory (DFT) at the B3LYP/6-31++G(d,p) basis set level were performed on five sym-triazines used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their percentage inhibition efficiencies. The results of the calculations and experimental IE% were subjected to correlation analysis and indicate that their inhibition effect are closely related to EHOMO, ELUMO, energy gap, hardness, softness, electronegativity and the fraction of electrons transferred were calculated. The theoretically obtained results were found to be consistent with the experimental data reported. (C) 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.