The effect of 4-amino-5-(4-pyridyl)-4H-1,2,4-triazole-3-thiol (4APT) on the corrosion of mild steel in hydrochloric acid solution was investigated as electrochemically, theoretically and qualitatively. Inhibition efficiency of 4APT on mild steel surface was observed and illuminated by EIS, potentiodynamic polarization, SEM and FTIR techniques. All of the experiments show that 4APT provides a good film layer on the surface and inhibits the corrosion of mild steel by adsorbing on a vast scale. The inhibition efficiency of 4APT was found approximately 96%. It is understood that 4APT is an effective inhibitor for acid corrosion of mild steel.