In a recent paper, a naphthalene-based azo-azometin inhibitor of 4-((4-chlorophenyl) diazenyl)-2-((naphthalen-1-ylimino)methyl) phenyl, HL, has been prepared by reaction of 1-(3-formyl-4-hydroxyphenylazo)-4-chlorobenzene with 1-naphthylamine in EtOH. The HL was characterized using NMR, FT-IR and Uv-Vis spectroscopic methods. The inhibitive effect of HL was investigated as corrosion inhibitor of mild steel in 0.1 M HCl solution using weight loss, polarization and electrochemical impedance spectroscopy techniques. The effects of time, temprature and concetration of HL on corrosion rate of mild steel was also investigated. The scanning electron microscopy, SEM, study of steel in HCl medium indicated that the inhibition is the result of the adsorption of HL on the mild steel surface and blocking of the active sites. The presence of N and O atoms, heterocyclic and aromatic rings, increase the inhibition effect of HL.