Abstract: Inhibition of mild steel corrosion by cetyl pyridinium chloride in hydrochloric acid was investigated using the weight loss method.It was found that corrosion inhibition increased with the increase in concentration of RNCl.The unchanged corrosion inhibition occurred when the concentration of RNCl reached 5×10-6.From the discussion results of adsorption model,it was found that in the temperature of 30℃ and 40℃,the adsorption model of RNCl on the mild steel surface obey the Langmuir adsorption equation.On the other 1and,in the temperature of 50℃ and 60℃,the adsorption model of RNCl on the mild steel surface obey the 1rumkin adsorption equation.The thermodynamic constants were obtained.The mechanism for corrosion inhibition was discussed with adsorption theory.