Lyophilized hot water extract of Purple Knight leaves (PKLE) has investigated as an environment friendly and cost effective corrosion inhibitor against corrosion of mild steel in 1 M HCl solution. Corrosion inhibition efficiency of PKLE has studied using Gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopic methods. PKLE acts as mixed inhibitor and its adsorption on mild steel surface in 1 M HCl solution follows the Langmuir adsorption isotherm. Thermodynamic parameters of the adsorption process and apparent activation energy for the corrosion process have been determined. Both physical and chemical adsorption contributes to the overall adsorption process. Scanning Electron Microscopy reveals a significant protection of metal surface by PKLE molecules. The chemical characterization of PKLE is carried out by gas chromatography and FT-IR spectroscopy.