he ureido substituted imidazolium bromides (code M-n) were synthesized and their anti-corrosion per formance on A3 steel in 5 M HCl solution was studied by weight loss test, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization. The experimental results reveal that the inhibitor are efficient mixed type corrosion inhibitors, and their inhibition efficiencies increase with increasing the concentration and alkyl chain. Thermodynamic parameters were calculated and discussed, indicating the adsorption of M-n on steel surface obeys the Langmuir adsorption isotherm and spontaneous exothermic process. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and quantum chemistry calculation further confirm the existence of an effective protective film of M-n on A3 steel surface.