The performance of rare earth 4-hydroxycinnamate (RE(4OHCin)₃) inhibitor in CO₂ corrosion has been studied by electrochemical and surface analyses. AS1020 and X65 steels with different microstructures have been used in a saturated CO₂ 0.01 M NaCl solution at room temperature and pH 4. The results showed that X65 steel has a better corrosion resistance than the AS1020 steel in saturated CO₂ 0.01 M NaCl solution. In contrast, RE(4OHCin)₃ has more effective carbon dioxide inhibition for AS1020 compared with X65. The same inhibition mechanism has been proposed for the formation of protective inhibiting deposits at the active electrochemical corrosion sites in addition to a thinner surface film deposit on the surface and RE(4OHCin)₃ mitigates corrosion by promoting random distribution of minor anodes.