A new compound of corrosion inhibitor namely 2-(2-hydroxyethyl)-6-phenylpyridazin-3(2H)-one (GP1) was synthesized and its inhibiting action on the corrosion of mild steel in 1.0 M hydrochloric acid at 308 K was investigated by various corrosion monitoring techniques. A preliminary screening of the inhibition efficiency was carried out using weight loss measurements. Potentiodynamic polarization and AC impedance methods have been used. Potentiodynamic polarization studies showed that this pyridazin derivative was mixed type inhibitor. The effect of temperature on the corrosion behaviour of mild steel in 1.0 M HCl with the addition of this compound was studied in the temperature range from 308-343K. The adsorption of this inhibitor on mild steel surface from hydrochloric acid obeyed the Langmuir adsorption isotherm.