The inhibiting behavior of 1-ethyl-3-butylbenzotriazolium ionic liquids，[C2Bt][Br] ,on mild steel corrosion in 5 wt.% HCl as corroding solution was investigated using weight loss，potentiodynamic polarization and electrochemical impedance measurements. The obtained results indicated that [C2Bt][Br] is a good inhibitor for the mild steel in 5 wt.% HCl solution. The inhibition efficiency increased with an increase of inhibitive concentration. Potentiodynamic polarization data indicated that the [C2Bt][Br] acted essentially as a mixed-type inhibitor. The electrochemical impedance study showed that corrosion inhibition took place by adsorption.