Corrosion is an unavoidable but a controllable process. Due to the issues of toxicity of substances, there is an increasing interest in exploration and utilization of eco-friendly inhibitors, which are also known as green inhibitors. The inhibitive effects of aqueous extracts of Opuntia ficus indica (OFI) on the corrosion of mild steel, in hydrochloric acid have been investigated by weight loss and polarisation measurements. Weight loss measurements gave an inhibition efficiency of about 91 % in the presence of 5g/l at 308K. EIS results revealed that green inhibitor effects excellently as a corrosion inhibitor for C38 steel in 1 M hydrochloric acid media with retarded the dissolution reactions to an extent dependent on the metal used. Polarisation measurements indicate that all the extracts increase the polarisation of the cathodic reaction of all the steel tested and the anodic polarisation, have no effect on the anodic reaction. The above results showed that OFI acted as a mixed-type corrosion inhibitor and the mechanism of inhibition has been discussed. The changes in impedance parameters (charge transfer resistance, Rct, and double layer capacitance, Cdl) were indicative of adsorption of OFI on the steel surface, leading to the formation of protective films. The adsorption of used compound on the steel surface obeys Langmuir’s isotherm. The apparent activation energy, enthalpy of the dissolution process and the free energy were determined and discussed.