Quaternized polyethyleneimine (QPEI) was prepared via two procedures of macromolecular reactions, tertiary amination reaction and quaterisation, and its chemical structure was characterized with infrared and UV spectra. In this paper, the corrosion inhibition property of QPEI for low carbon steel (A(3) steel) in H2SO4 solution was mainly investigated by using weight loss method, electrochemical technique (potentiodynamic polarization curves) and scanning electron microscopy (SEM) investigation. Meanwhile, the corrosion inhibition mechanism of QPEI was also fully explored. The experiment results show that as a polymeric quaternary ammonium salt, QPEI is quite an excellent corrosive inhibitor, it possesses outstanding inhibition action for low carbon steel, and only with 5 mg L-1 of QPEI concentration, the inhibition efficiency of A(3) steel can reach to 92% in 0.5 M H2SO4 solution for 72 h of immersing time. The inhibition efficiency increases with the increase of QPEI concentration, and the inhibition efficiency also increases with the enhancement of the cationic degree (quaternized agree) of QPEI. QPEI acted as a mixed type inhibitor with anodic predominance. QPEI possesses functions of double filming, adsorption filming and polymer filming, so with the cooperation of the two filming effects, QPEI can form a compact barrier film on A(3) steel surface, and prevent metal dissolution and retard H+ discharging. The adsorption of QPEI on A(3) steel surface was found to follow the Langmuir isotherm equation. (C) 2007 Elsevier B.V. All rights reserved.