The inhibitive effect of Thevetia peruviana on the corrosion behavior of carbon steel in 1 M HCl was studied using the weight loss method, polarization, AC electrochemical impedance spectroscopy and electrochemical frequency modulation techniques. The inhibition efficiency was found to depend on the concentration of the extract and the temperature of the medium. Results showed that this extract acts as a mixed-type inhibitor. The charge transfer resistance increases but the double layer capacitance decreases by increasing the extract concentration. It was found that the investigated extract acts via adsorption on the carbon steel surface and obeys the Temkin adsorption isotherm. The effect of temperature on the corrosion rate in the absence and presence of the extract was investigated, some activated, and the adsorption parameters were calculated. The results of the electrochemical methods were in good agreement with the weight loss measurement results