The inhibitive effect of Rosmarinus officinalis on the corrosion behavior of carbon steel in 1M HCl was studied using weight loss method, polarization, ac electrochemical impedance spectroscopy (EIS), electrochemical frequency modulation (EFM) techniques and surface examination. The inhibition efficiency is found to depend on the concentration of the inhibitor extract and the temperature of the solution. The mechanism of inhibition was deduced in terms of blocking the electrode surface by the adsorption of the inhibitor extract molecules through the active centers in their chemical structures. The adsorption process typically corresponds to Langmuir adsorption isotherm. The effect of temperature on the corrosion rate in the absence and presence of the inhibitor extract was investigated and some activated thermodynamic parameters were calculated.