The marine macroalgae extract of caulerpa prolifera in aqueous 1 M HCl was systematically investigated to ascertain its inhibitor effect on corrosion of mild steel and its mechanism of the inhibition by electrochemical and weight loss methods. The results revealed a high inhibitive effect of C. prolifera. The inhibition efficiency was found to increase with increasing concentration of the algal extract but decreased with increase in temperature which is attributed to the physisorption. Potentiodynamic polarization measurement showed that C. prolifera extract acted as mixte type inhibitor. The increase in activation in energies of corrosion process in presence of the extract indicates that C. prolifera retarded the rate of corrosion of mild steel in 1 M HCl solution. The nature of adsorption of the C. prolifera on mild steel surface was in conformity with Langmuir isotherm. The results obtained with different method are in good agreement.