The inhibition effect of Sulfamethoxypyridazine(SMP) against mild steel corrosion in 2.0 M H3PO4 was evaluated using weight loss and electrochemical techniques (potentiodynamic polarization and electrochemical impedance). The experimental results showed that SMP is a good corrosion inhibitor and the inhibition efficiency increased with the increase of SMP concentration, while the adsorption followed the Langmuir isotherm. Theoretical calculation was carried out to establish mechanism of corrosion inhibition for mild steel with SMP in 2.0 M H3PO4 medium. The inhibition action of the compound was assumed to occur via adsorption on the steel surface through the active centers in the molecule. The corrosion inhibition is due to the formation of a chemisorbed film on the steel surface.