The inhibition efficiency of Rosmarinus officinalis leaves extract (ROE) on the corroded mild steel in 1N Phosphoric acid has been investigated by the weight loss method and Potentiodynamic polarization methods at 303K-333K. The concentration of inhibitor increased with decreased in the corrosion rate compared to blank, it was also found that the inhibition efficiency increased synergistically in the presence of Potassium iodide. A Potentiodynamic polarization method shows that the studied inhibitor is the mixed type. The adsorption of inhibitor on mild steel surface has been found to obey’s Temkin’s, Frumkin, adsorption isotherm. Thermodynamic parameters reveal that the spontaneous process and exothermic in nature. The Surface morphology on the mild steel surface analyzed by FT- IR and SEM analysis. The electronic properties such as EHOMO, ELUMO, energy gap orbital’s using Density Functional Theory at the B3LYP/6-31G ++ (d, p) basis set with the inhibiting action of Rosmarinus Officinalis. Quantum chemical calculations confirmed the adsorption of the molecule on the surface.