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Carbon nanotube (CNT) has excellent properties as a coating material. However, most methods for CNT coating have limitations. Recent studies have shown that polydopamine (PDA) can be used as a good and eco-friendly corrosion resistance coating material. However, the effect of PDA/CNT nanocomposite coating on the corrosion mitigation property of carbon steel (CS) remains unknown. Therefore, the objective of this study was to determine the effect of PDA/CNT nanocomposite coating on the corrosion mitigation property of carbon steel (CS). A dopamine (DA)/CNT composite was first synthesized. CNT length was controlled by sonication. Based on dynamic light scattering, field emission transmission electron microscopy, and atomic force microscopy analyses, average lengths of CNTs in long and short DA-CNTs were 500 and 50 nm, respectively. Carbon steel specimens were subjected to corrosion resistance analysis using electrochemical methods. Results of electrochemical tests showed that corrosion resistance of CS coated with PDA and PDA/CNTs were dramatically higher than those of bare CS. This corrosion mitigation was attributed to two mechanisms. First, PDA decreased the corrosion rate due to the generation of a thin and dense layer on the surface. Second, CNTs suppressed the cathodic partial reaction due to oxygen adsorption at defect sites of CNTs. Keywords: Polydopamine (PDA), Carbon nanotube (CNT), Corrosion, Coating, Nanocomposite, Electrochemical analysis