5-Alkyl-4 phenyl-4H-[1,2,4]triazole-3-thiol of phenyl hydrazides of fatty acids from Neem, Rice bran and Karanja oils have been synthesized and evaluated as corrosion inhibitors of mild steel (MS) in hydrochloric acid solutions by weight loss method. The adsorption of all the phenyl hydrazides derivatives on mild steel surface in the acid solution has been found to obey Langmuir's adsorption isotherm. Inhibition efficiency of these compounds has been found to vary with concentration of the compound, solution temperature, immersion time and concentration of acid solution. The activation energies (E(a)) have been calculated to investigate the mechanism of the corrosion inhibition. The inhibition efficiency increases with the increase in concentration of the inhibitors.