The 5-chloro-benzimidazol-2-one (B1), 5-methyl-benzimidazol-2-one (B2) and benzimidazol-2-one (B3) have been investigated for mild steel corrosion in 1 M HCl at different concentrations using weight loss measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. Generally, inhibition efficiency of the investigated compounds was found to depend on inhibitor concentration and their structures. A comparison results showed that B1 was the best inhibitor. For this, a detailed study was making for this compound using electrochemical measurements. Potentiodynamic polarization studies clearly reveal that B1 acts essentially as mixed inhibitor. The inhibition efficiency increases with immersion time and concentration. In addition, at 24 h, the EIS showed the formation of a protective film. The adsorption of B1 on mild steel surface is found to obey Langmuir adsorption isotherm.