The B/N pairs co-doping (5, 0) single-walled carbon nanotubes (SWCNTs) have been investigated by using density functional theory. We gradually increase B/N doping concentration to simulate the growth of B/N pairs doping. We find that B/N pairs prefer to form a B/N hexagonal ring and then B/N rings successively grow around the axis until they are end to end. All B/N pairs doped tubes are turned to semiconducting and the five BN rings co-doped (5, 0) tube shows the occurrence of magnetism. Moreover, the increase of the doping concentration in a particular law may not change electrical properties obviously. (C) 2016 Elsevier B.V. All rights reserved.