Single atom (B, N or P) doping zigzag single-walled carbon nanotubes (SWCNTs) evolve from semiconducting to metallic by using density functional theory. The energy gaps of (8,0) tubes significantly narrow as a result of B/N and B/P pair co-doping. Especially, B/P pair co-doping makes the bottom of conduction band move to the Fermi level direction, and the top of valence band barely shift. With the increasing concentration of B/P pair, the bottom of conduction movement becomes stronger and the energy gaps decrease. Moreover, B or N-rich doping significantly changes the semiconducting character of pure B/N pairs co-doped models. (C) 2016 Elsevier B.V. All rights reserved.