The inhibition effect of 8-(4-chlorophenyl)-3-hydroxy-4,6-dioxo-2-(p-toly)-4,6-dihydropyrimido [2,1-b] [1,3] thiazine-7-carbonitrile (HPDTC-1), 3-hydroxy-4,6-dioxo-8-phenyl-2-(p-toly)-4,6-dihydropyrimido [2,1-b] [1,3] thiazine-7-carbonitrile (HPDTC-2) and 3-hydroxy8-(4-nitrophenyl)-4,6-dioxo-2-(p-toly)-4,6-dihydropyrimido [2,1- b] [1,3] thiazine-7-carbonitrile (HPDTC-3) on the corrosion behavior of carbon steel in hydrochloric solution were investigated by experimentally using polarization, electrochemical impedance spectroscopy and gravimetric measurements and complementary with density functional theory (DFT) methods. The results of this study show that these compounds give excellent inhibiting properties for carbon steel corrosion in acidic medium with the efficiency that exceeds 96% for HPDTC-3, 95% for HPDTC-2 and 92% for HPDTC-1 at 10-3 mol/L. Polarization curves prove that these organic compounds act as inhibitors mixed-type. The adsorption of all studied inhibitors on the carbon steel surface follows Langmuir isotherm model. The variation of corrosion currents at different temperatures we lead to determination of activation parameters of the dissolution process and from the Langmuir adsorption isotherm we have calculate the thermodynamic adsorption parameters of the tested inhibitors. For establish the correlation between experimental data and theoretical results, some quantum chemical parameters for the three compounds were evaluated for provide more idea on the mechanism of inhibition of the corrosion process.