The corrosion inhibition efficiency of pyrimidothiazine compound, namely: 8-ethyl-3-hydroxy-4,6-dioxo-2-(p-tolyl)-4,6-dihydropyrimido [2,1-b][1,3] thiazine-7carbonitrile (EHDPTC), was evaluated in the system steel/1 M HCl. This compound is investigated experimentally using weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy. The results showed that the inhibition mechanism involves blockage of the steel surface by the inhibitor molecules by a Langmuir-type adsorption process and that the structure of molecule plays an important role in the inhibition efficiency of the synthesized inhibitor. The thermodynamic parameters of activation and adsorption have been calculated and discussed in detail. Furthermore, the quantum chemical parameters have been calculated and discussed in view of the results earlier reported. © 2018 by CEE (Center of Excellence in Electrochemistry).