The article presents the results of studies on the synthesis of new 1,3-benzoxazine derivatives and a study of their corrosion inhibition properties in a sulfuric acid media. The synthesis of 1,3-benzoxazine was carried out by triple condensation of 4-iso-decylphenol with formaldehyde and benzylamine at 95-98°C for 3.5 h’s. The yield was 80%. Then, the obtained compound was subjected to quaternization with allyl bromide at 55-58 °C for 3.5 h’s to obtain N-allyl 1,3-benzoxazinium bromide, thatcontaining a quaternary nitrogen atom and Br anion in its structure. Their structures were confirmed by NMR and IR spectroscopy data. The obtained compounds were studied as inhibitors of acid corrosion of unalloyed steel St.3 by the gravimetric method in 0.5 N H2SO4. The effect of temperature (25, 40, and 60°C) and concentration (50, 75, and 100 mg/L) on the corrosion rate (K, g/m2h) and corrosion protection degree (Z, %) was studied. It was found that at a temperature of corrosion process 25°C and at the concentration of compounds I and II 50 mg/L, protection degrees were 88% and 95.6%, respectively. By the increasing temperature to 40°C and then to 60°C, the protective effect decreased slightly. At 60°C and a maximum concentration of compounds I and II of 100 mg/L, protection degrees were 83.3% and 90.0%, respectively.