Calculations have been made for zigzag carbon nanotubes containing substitutional B/P pair and Al/N pair using density functional theory. We study seven possible models for B/P pair and Al/N pair doping into nanotubes. And all doped models are semiconductors. Moreover, we study the law of band gaps and analysis the reason in terms of the symmetry of structures. Our results indicate that the structures will get same destruction when the impurities atoms pair exists in similar profile and atmosphere. The same destruction leads to same band gaps. (C) 2014 Elsevier B.V. All rights reserved.