Display omitted •GO with different aspect ratios were prepared and the uniform GO/epoxy composite coatings were fabricated.•Anti-corrosion performance of composite coatings containing GO sheets with different aspect ratio were investigated.•Anti-corrosion mechanism of different GO/epoxy composite coatings were studied.•The aspect ratio of GO sheets was a significant factor for enhancing corrosion resistance of GO/epoxy coatings. Graphene oxide (GO) has a wide application prospect in the field of metal protection due to its good dispersion, chemical activity and physical barrier property. Herein, graphene oxide sheets with different aspect ratio were first prepared by simply controlling the procedure of chemical exfoliation and then used to fabricate GO/epoxy composite coatings. The morphology and structural features of as-prepared GO sheets were characterized by Raman spectroscopy, X-ray diffraction and atomic force microscopy. The dispersion and anticorrosive performances of GO with different aspect ratios incorporated in epoxy-based waterborne composite coatings were investigated by Raman spectroscopic mapping, electrochemical measurements and scanning vibrating electrode technique. The results demonstrated that the different types of GO showed homogeneous dispersion in epoxy resin and GO with higher aspect ratio exhibited larger corrosion protection potential. Furthermore, the anti-corrosion mechanisms for composite coatings enhanced via GO with different aspect ratio were tentatively proposed, which indicating that the GO sheets with higher aspect ratio provided a more tortuous permeation path for corroding medium and thus displayed better corrosion resistance.