and N co-doped armchair (5,5) single walled carbon nanotubes with different concentrations of the B-N pairs were studied with the first principle density functional theory B3LYP/3-21G(d). The periodic boundary conditions were employed to simulate the ultra long tube model. The structure parameter, energy, energy gaps and the band structures were obtained. The doping atomic pair ratios x for 1/160-1/10 were examined and the results show that the energy changes have excellent relationships with the ratio. The energy gaps are within 0.240-0.580 eV showing semiconductor conductivity and having direct gaps. (c) 2012 Elsevier B.V. All rights reserved.