The work aims to prepare an effective and economical inhibitor of phenanthroline derivative. The corrosion inhi-bition ability of 1,10-phenanthroline (PHEN) and its derivative 2-phenyl-1H-imidazo[4,5-f][1,10] phenanthroline (PIPH) on mild steel in HCl solution was investigated at various concentrations and temperatures by weight loss, polarization curves and electrochemical impedance spectroscopy. The surface morphology of mild steel without and with different corrosion inhibitors was observed by scanning electron microscope (SEM) and then the relationship between the molecular structure of PHEN and PIPH and their inhibitory properties was discussed with theoretical calculations. Inhibition efficiency of the PHEN and PIPH increased with the increase of concentration, but decreased as the temperature increased. In the same condition, the corrosion inhibition of PIPH was better than that of PHEN. The PHEN and PIPH showed the maximum inhibition efficiencies of 97.4% and 99.5% at 30 ℃ with the concentration of 1 mol/L. PHEN and PIPH obeyed the Langmuir adsorption isotherm on metal surface and was respectively ?21.44 and ?24.28 kJ/mol. From the results of electrochemical experiment, both inhibitors were the mixed-type inhibitor mainly focusing on cathodal depression. The SEM observations showed that the inhibitors could inhibit corrosion of mild steel in HCl effectively. The inhibitors PHEN and PIPH show excellent corrosion performance in 1.0 mol/L HCl and PIPH with multiple adsorption centers has better inhibition property than PHEN.