The stability and electronic structure of single-walled carbon nanotubes with B/N co-doping are investigated in detail by using the first-principles theory. From eight possible B/N co-doping configurations, it is found that the one with substitutional B and N atoms located at neighboring sites has a smaller formation energy than that with separate B/N atoms. The electronic structure of armchair carbon nanotubes evolves from metallic to semiconducting as a result of B/N co-doping, whereas the energy gaps of the intrinsically semiconducting nanotubes are reduced significantly. In contrast, the small zigzag nanotubes always remain metallic properties due to their large curvature effects except (5, 0) after B/N co-doping. As the atomic concentration of B/N co-doping is increased, the energy gaps of carbon nanotubes oscillate around a constant level, which is much lower than the energy gap of BC(2)N nanotubes. Moreover, the B/N co-doped carbon nanotubes with B- or N-rich impurities exhibit the characteristics of an acceptor or donor, respectively, since their electronic structures are significantly influenced by the occupied states in the valence and conduction bands due to the shifting of the Fermi level.