CETYLTRIMETHYL ammonium bromide (CTAB) was investigated …….as corrosion inhibitors of mild steel in 0.1 and 0.5 M HCl solutions using weight loss, Tafel polarization and electrochemical impedance spectroscopy techniques. The experimental results showed that the inhibition efficiency increases with increasing of CTAB concentrations but decreases with increasing temperature. The adsorption of CTAB on the mild steel surface obeyed the Langmuir adsorption isotherm. Potentiodynamic polarization curves showed that CTAB acted as a cathodic inhibition predominantly in hydrochloric acid. This was supported by the impedance measurements which showed a change in the charge transfer resistance (Rct) and double layer capacitance (Cdl) indicating adsorption of CTAB on the mild steel surface. Thermodynamic parameters were also calculated to know the mechanism of the inhibition. The thermodynamic functions of the adsorption processes were calculated from the weight loss and effect of temperature data and were used to analyze the inhibitor adsorption mechanism. The thermodynamic parameters indicate that physical adsorption and chemisorptions of the inhibitor on the metal surface may occur.