In this study, the inhibition effect of 6-phenylpyridazin-3(2H)-one (GP0) on mild steel corrosion in 1.0M HCl solution was studied. For this aim, electrochemical techniques such as potentiodynamic polarization curves, weight loss (WL) and electrochemical impedance spectroscopy (EIS) were used. It was shown that, the GP0 base has remarkable inhibition efficiency on the corrosion of mild steel in 1.0M HCl solution. Polarization measurements indicated that, the studied inhibitor acts as mixed type corrosion inhibitor with predominantly control of cathodic reaction. The inhibition efficiency depends on the concentration of inhibitor and reaches 82.7% at 1.0mM GP0. The remarkable inhibition efficiency of GP0 was discussed in terms of blocking of electrode surface by adsorption of inhibitor molecules through active centers. The adsorption of GP0 molecules on the mild steel surface obeys Langmuir adsorption isotherm.