First-principles calculations are employed to investigate total energies and electronic structures of the B/N doped silicon nanowires, the B/N doped silicon nanowires with and without dangling bond (DB). And the calculation indicates that the DB would lead to the doping failure. Band-structure calculations indicate that B/N doped silicon nanowires without dangling bond show regular p/n type of the charge carrier, while the dangling bond would cause signal atom doping failure, which is not due to the transfer of electrons, but results from the capturing of the electron (hole) by the defect energy level induced by the surface dangling bond. Moreover, the small molecule adsorption can reactivate impurities doping p/n characteristics. The reactivation mechanism is not the transfer of the electrons, thus it can hold the doping characteristics.