The inhibition effect of 2H-benzo[b][1,4]thiazin-3( 4H)-one (T1) on the corrosion of mild steel in 0.5 M H2SO4 was studied by weight loss, electrochemical impedance s pectroscopy (EIS) techniques and potentiodynamic po larization methods. The results showed that (T1) was a good in hibitor in 0.5 M H2SO4 and inhibition efficiency in creases with (T1) concentration to attain 91% at 10-3M at 308 K. E(%) values obtained from various methods used are reasonably good agreement. The adsorption of (T1)ob eyed the Langmuir adsorption isotherm. Polarization curves showed that (T1) acted as a mixed-type inhibitor in sulfuric acid. Quantum chemical approach used to calculate electronic properties of the molecule to ascertain the relation between inhibitive effect and molecula r structure.