Rosmarinus officinalis essential oil obtained by hydrodistillation was tested as corrosion inhibitor of steel in 0.5 M H2SO4 using weight loss measurements,and electrochemical polarisation methods.Results obtained indicate that the corrosion rate is reduced and R.officinalis oil adsorbs on the metal surface and then inhibits corrosion process.Its inhibition effciency increases with oil increasing content to reach 61% at 1 g/L.Polarisation curves indicate that RO acts as a cathodic inhibitor.The inhibition effciency of naturally oil remains slightly constant with the rise of temperature.Chemical analysis of R.officinalis essential oil was carried out using capillary GC and GC/MS.The oil composition was characterized by high amount of oxygenated monoterpene(77%) with 1,8-cineol(52.1%) as main component.Discussion of adsorption was investigated on the basis of essential oil composition.Finally,the isotherm adsorption of essential oil on the steel has been also reported. Rosmarinus officinalis essential oil obtained by hydrodistillation was tested as corrosion inhibitor of steel in 0.5 M H2SO4 using weight loss measurements, and electrochemical polarisation methods. Results obtained that that corrosion rate is reduced and R. officinalis oil adsorbs on the metal surface and then inhibits corrosion process. Its inhibition effciency increases with oil increasing content to reach 61% at 1 g / L. Polarisation curves indicate that RO acts as a cathodic inhibitor. The inhibition effciency of naturally oil remains slightly constant with the rise of temperature. Chemical analysis of R. essential oil essential oil was carried out using capillary GC and GC / MS. The oil composition was characterized by high amount of oxygenated monoterpene (77%) with 1,8-cineol (52.1%) as main component. Disacssion of adsorption was on the basis of essential oil composition. Finally, the isotherm adsorption of essential oil on the steel has been also reported.