Abstract: We studied boron -doped (2,2) single-walled carbon nanotubes(SWCNT) by using the first-principle method based on density functional theory. In this paper，Geometry structure, formation energy, band structure and density of states(DFT) are calculated. The results show the diameter of SWCNT will be increased when boron is adulterated, the process of boron doping SWCNT will release energy, it is feasible to substitute a carbon atom with a boron atom in SWCNT. It is also found that Fermi energy move to valence band, and B-doped carbon nanotubes is n type semiconductor.