Inhibition of copper corrosion by imidazole (IM) and 1-n-undecyl-imidazole (UDIM) in 0.5 M HCl was investigated by weight-loss measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). 1-n-undecyl-imidazole showed the better inhibitive effect compared with IM. Potentiodynamic polarization results revealed that both IM and UDIM acted as anodic inhibitors, in particular UDIM suppressed anodic current densities significantly. Molecular structure parameters of IM and UDIM were obtained by using an MM2 forcefield program and AM1 quantum chemical calculation. It was found that UDIM has higher levels of HOMO and LUMO energy and a larger electron density in its imidazoly ring.