A newly amino acid ionic liquid, 1 - octlyl-3-methylimidazolium L-prolinate ([Omim]Lpro) was investigated as the inhibitor for mild steel in 0.5 M H2SO4 solution using weight loss method, electrochemical measurements, scanning electron microscopy (SEM), and quantum chemical calculation. The obtained results revealed that [Omim]Lpro was a mixed-type inhibitor with a predominantly cathodic action for mild steel in 0.5 M H2SO4 solution, and inhibition efficiency reached nearly 80% at the concentration of 10mM, in which the Omim cation played a major role in the corrosion inhibition of [Omim]Lpro. The adsorption of [Omim]Lrpo on the mild steel surface was found to obey the El-Awady thermodynamic-kinetic model and Flory-Huggins isotherm equations; thus the thermodynamic and kinetic parameters governing the adsorption process were calculated and discussed. Moreover, quantum chemical calculation gave futher insight into the mechanism of inhibition of [Omim]Lpro.