Abstract The effects of carbon nanotube (CNT)–epoxy nanocomposite coating applied to glass fibre surface on tensile strength of single glass fibres are evaluated at different gauge lengths. The crack healing efficiencies obtained using two different types of CNTs with different structures, morphologies and dispersion characteristics in various concentrations are specifically studied. The results indicate that the tensile strength of single fibres increased significantly with increasing CNT content up to a certain level, depending on the type of CNTs. The crack healing efficiency was much higher for the fibres coated with straight, less entangled CNTs than those with highly entangled CNTs, indicating the CNT dispersion state in the coating played an important role. A strong correlation is established between the CNT dispersion state, the tensile properties of nanocomposite and the tensile strengths of fibres with the nanocomposite coating.