Boron nitride (BN) films are deposited onto Si substrates by plasma-assisted chemical vapor deposition. It is found that a variation in the surface roughness occurs with increasing deposition time. Ultraviolet photoelectron spectroscopy analysis reveals that negative electron affinity (NEA) appears on the surface of BN films treated with H-2 plasma and that NEA is maintained even after O-2 plasma treatment. NEA is lost by annealing at 1100 degrees C. Field emission characteristics of BN films with various surface roughnesses suggest electron emission due to Fowler-Nordheim tunneling from the energy state in the band gap. A turn-on electric field as low as 8 V/mu m is obtained for the H-2 plasma treated BN film.