Abstract Carbon nanotubes (CNTs) could be an effective additive for polymeric anticorrosion coatings due to their electrical conductivity. For enhancement of the interfacial compatibility and dispersion homogeneity of CNTs in the polymer matrix, functionalization of CNTs with matrix polymer is attractive. In this work, polybenzoxazine (PBz) as the matrix polymer is chemically bonded to multiwalled CNT (MWCNT) without altering the benzoxazine reactive groups through the radical transfer and addition reaction. The PBz-functionalized MWCNT (PBz-MWCNT) has been carried out with FTIR, XPS, and Raman spectroscopies. The PBz fraction of PBz-MWCNT is also quantitatively and qualitatively observed with thermogravimetric analysis and high resolution TEM, respectively. PBz-MWCNT has been utilized as an additive for crosslinked PBz based anticorrosion coatings for cold roll steel. An effective protection efficiency of 99.91% with an ultralow corrosion rate of 0.179 μm y−1 is recorded with the anticorrosion coating with 3.0 wt% of PBz-MWCNT. As the neat crosslinked PBz based anticorrosion coating shows a 90.28% protection efficiency and a 20 μm y−1 corrosion rate, the effectiveness of PBz-MWCNT on enhancement of the anticorrosion coating of crosslinked PBz is demonstrated.