1-dodecyl-3-methyl imidazolium chloride ([C12mim]Cl) was adsorbed on iron surface to form an inhibitive film. The adlayer inhibited the metal corrosion in 0.2 M H2SO4 efficiently. The inhibitive effect of [C12mim]Cl was characterized by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Impedance spectra demonstrated that the values of charge-transfer resistance increased when adding [C12mim]Cl into 0.2 M H2SO4. The inhibition efficiency increased with increasing the concentration of [C12mim]Cl. Scanning electron microscopy (SEM) analysis showed that the corrosion of iron surface had been inhibited by [C12mim]Cl.