The adsorption and inhibition effect of 1,1'-thiocarbonyldiimidazole (TCDI) on the corrosion of mild steel (MS) in 0.5 M HCl solution was studied in both short and long immersion time (120 h) with the help of electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques. For long-time tests, the hydrogen gas evolution (VH2-t) and the change of open circuit potential with immersion time (E (ocp)-t) were also utilized in addition to the former two techniques. The surface morphology of MS after its exposure to 0.5 M HCl solution with and without 1.0 x 10(-2) M TCDI was examined by scanning electron microscopy (SEM). It was demonstrated that the inhibition efficiency of studied inhibitor is concentration depended and increased with TCDI concentration. The higher value of inhibition efficiency was obtained after longer immersion time merely on the basis of strong increase of corrosion rate of mild steel in the blank solution. The high inhibition efficiency was discussed in terms of adsorption of inhibitor molecules and protective film formation on the mild steel surface which was substantiated by SEM micrographs. The adsorption of TCDI on MS was found to obey Langmuir adsorption isotherm.