A new phenanthroline derivative, 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (MEIP), was synthesized and characterized by elemental analysis, FT-IR, (HNMR)-H-1, and (CNMR)-C-13 spectra. MEIP was evaluated as corrosion inhibitor for carbon steel in 0.5 M H2SO4 solution using gravimetric and UV visible spectrophotometric methods at 303-333 K. Results obtained show that MEIP is a good inhibitor for mild steel in H2SO4 solution. The inhibition efficiency was found to increase with increase in MEIP concentration but decreased with temperature, which is suggestive of physical adsorption mechanism. Activation parameters and Gibbs free energy for the adsorption process using Statistical Physics were calculated and discussed. The UV visible absorption spectra of the solution containing the inhibitor after the immersion of mild steel specimen indicate the formation of a MEIP-Fe complex. The calculations of global reactivity indices of MEIP such as the localization of frontier molecular orbitals, E-HOMO, E-LUMO, energy gap (Delta E), dipole moment (D), hardness (eta) softness (sigma), the fractions of electrons transferred (Delta N), electrophilicity index (omega), total energy change (Delta E-r), and Mulliken charge distributions together with local reactivity by means of Fukui indices were used to explain the electron transfer mechanism between the MEIP molecules and the steel surface. The quantum chemical calculations were performed at the density functional theory (DFT) level using B3LYP functional with the 6-31G (d) basis set for all atoms using Spartan'06 V112 program package.