The inhibition behavior of low carbon steel in 1 M HCl by l-tryptophan was investigated with weight loss experiment and Tafel polarization curve in the used temperature range (298-328 K). All the experimental results show that l-tryptophan has excellent corrosion inhibition performance and the most effective concentration of inhibitor is 1 x 10(-2) mol L-1. The Tafel polarization curve results indicate that l-tryptophan acts more as a cathodic than anodic inhibitor. The adsorption of l-tryptophan on the surface of low carbon steel obeys the Langmuir adsorption isotherm, and the thermodynamic parameters were determined and discussed. The adsorption behavior of l-tryptophan at Fe surface (1 1 0) was also investigated by the molecule dynamics simulation method and density functional theory. The results indicated that the l-tryptophan could adsorb firmly on the Fe surface through the indole ring with pi-electrons and nitrogen/oxygen atom with lone-pair electrons in its molecule.