Steel tube bases used for signposts are prone to accelerate the corrosion near the boundary with ground due to crevice corrosion. Carbon fiber reinforced plastic (CFRP) is used to repair such corrosion in steel tube structures. For CFRP bonded repairs and strengthening, it is critical to ensure proper bond length and prevent delamination of CFRP to achieve effective repair and reinforcement. In this study, shear-lag theory is applied to CFRP-bonded cantilever steel tubes to derive the shared bending moments in the steel tube and the CFRP, as well as the shear stress in the adhesive. The derived equations were validated by comparison with finite element (FE) analysis. In addition, the study proposes a required bond length for CFRP bonded repairs and strengthening, which was confirmed by FE analysis of ribbed steel tubes.