The corrosion behavior of mild steel (Ck 45) in 3.0 M H3PO4 solution in the absence and presence of penicillin G was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The best inhibition effect at 10 mM of drug was a marked characteristic of the inhibitor. It was found that the inhibition efficiency of the inhibitor depends on its concentration and chemical structure. In other words, the inhibition efficiency increases with the inhibitor concentration. The adsorption process obeyed Langmuir adsorption isotherm. Some thermodynamic parameters were determined from the effect of temperature on corrosion and inhibition processes. Potentiodynamic polarization measurements indicated that the drug was a mixed type inhibitor.