The electronic structures and spin polarization characteristics of pure and Mn-doped closed zigzag GaN nanotubes are theoretically investigated. Owing to the hybridization of Mn 3d orbitals and N 2p orbitals, the spin-polarized electron current mainly comes from the contributions of the Mn atom and the nearest neighbor N atoms. The tunneling probability of spin-polarized electrons is sensitively dependent upon the corresponding doping states under the external electric field. We suggest that Mn-doped zigzag GaN nanotubes with Ga-riched caps, rather than Mn-doped armchair ones, could be used as a good candidate for the nanoscale spinpolarized field emitter. (C) 2004 Elsevier B.V. All rights reserved.