The electronic structure of two ultra small diameter metallic single-walled carbon nanotubes (SWCNTs) with B/N co-doping are investigated by using the first-principles theory. The results show that the B/N co-doping configuration is the energetically stable structure. The electronic structures of ultra small diameter metallic SWCNTs evolve from metallic to semiconducting as a result of B/N co-doping, in most case the energy gaps increase with increasing the symmetry destruction factor delta. In addition, we also discuss the electronic structures of B/N co-doped SWCNTs with B- or N-rich impurities, which are converted into metal from semiconductor due to B- or N-impurities. (C) 2013 Elsevier B.V. All rights reserved.