Hexagonal polycrystalline boron nitride (BN) films are synthesized on Si substrates by plasma assisted chemical vapor deposition. In the case of the BN films thicker than 20 nm, the turn-on electric field of the electron emission is strongly influenced by the surface roughness rather than the film thickness. On the other hand, in the case of the BN film with a thickness of 8 -10 nm, it is found that, the turn-on electric field as low as 8.3 V/mum is achieved in spite of the surface of the BN nanofilm being flat as well as the Si substrate. A significant reduction in the effective potential barrier height is suggested. The tunneling controlled field emission is proposed for the BN nanofilm with positive space charge. The BN nanofilm is deposited onto the graphite nanofiber sample. A significant improvement of the field emission characteristics is demonstrated. (C) 2003 American Vacuum Society.