The inhibition effect of 5-amino 1,3,4-thiadiazole-2-thiol (5-ATT) on the corrosion of mild steel in 1 M HCl solution at 303 K was studied practicing weight loss, electrochemical electrochemical (AC and DC) studies and Xray photoelectron spectroscopy (XPS) methods. The obtained results showed that 5-ATT was an effective inhibitor in HCl medium. The inhibition efficiency, eta(%), increased with the increase in concentration inhibitor but the temperature has almost no effect on the inhibition efficiency of 5-ATT. The activation dissolution thermodynamic parameters were calculated and discussed. Thermodynamic data and XPS analysis clearly show that the adsorption mechanism of 5-ATT on the mild steel surface in 1 M HCl solution is mainly electrostatic-adsorption. Tafel curves reveal that 5-ATT behaves as a mixed-type inhibitor with cathodic predominance and obeyed Langmuir adsorption isotherm. With the aim of extracting the parameters of the electrochemical impedance spectroscopy an appropriate equivalent circuit model was offered.