The inhibition ability of Hexamethylenediamine penta(methylphosphonic) acid (HTMP) against carbon steel corrosion in 1 M HCl at 30°C was evaluated by weight loss, electrochemical (potentiodynamic and electrochemical impedance spectroscopy, EIS) methods. The experimental results showed that HTMP was a good inhibitor for the steel corrosion in 1 M HCl medium and its inhibition efficiency increased with the inhibitor concentration. Data, obtained from ac impedance measurements, were analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. Adsorption of HTMP on the carbon steel surface followed the Langmuir adsorption isotherm. Surface analysis by (SEM) supported the formation of a protective inhibitor film on the carbon steel surface. Furthermore, the theoretical study was carried out using the density functional theory (DFT) method