This paper describes the inhibitory effect of 1,4-dihydro-6-methyl-quinoxaline-2,3-dione(P2)on corrosion of mild steel in aqueous 1 M HCl .The inhibition behavior of P2 was investigated using different techniques (weight loss method, potentiodynamic polarisation technique and electrochemical impedance spectroscopy (EIS)).The nature of adsorption of compound P2 on mild steel surface is found to obey Langmuir adsorption isotherm. EIS measurement result is also correlated with the result of polarization. EIS plots exhibit individual capacitive loop. The presence of P2 in 1 M HCl solution increases charge transfer resistance, while it reduces double layer capacitance values. DFT study confirmed the adsorption of inhibitor molecules on mild steel surface.