Boron nitride (BN) nanohorns were synthesized by an arc-melting method, and atomic structure models for BN nanohorns with tetragonal BN rings were proposed from high-resolution electron microscopy and image calculation. Stability and electronic structures of the BN nanohorns were investigated by molecular orbital calculations, comparing with carbon nanohorns. The calculation showed that multi-walled BN nanohorns would be stabilized by the stacking of BN nanohorns. An energy gap of BN nanohorn was calculated to be 0.80 eV, which is lower compared to those of BN clusters and nanotubes.