The performance of a natural corrosion inhibitor made from grapefruit skin, which was extracted by heat-reflux-extract method, has been investigated by weight loss measurements, potentiodynamic polarization curves for hot rolled carbon steel in 2 mol/ L Hydrochloric acid solution, within the range of 30 ~50 ℃. It is revealed that grapefruit skin extract acts as a good corrosion inhibitor. The inhibition efficiency increases with concentration of extract and temperature, and reaches to the maximum value as 78. 4% at 50 ℃. The adsorption of the extract on carbon steel surface obeys the Langmuir’s adsorption isotherm, mainly attributes to physical adsorption. The potentiodynamic polarization curves indicate that the extract mainly inhibits cathodic hydrogen evolution reactions and acts as a mixed type inhibitor.