Addition of carbon atoms into hexagonal boron-nitride film was attempted to solve serious issues of cracking or peeling off of the substrate. Boron-carbon-nitride (BCN) nanofilms are synthesized on n-type Si(100) substrates by plasma-assisted chemical-vapor deposition with a horizontal quartz reactor. Field-emission characteristics are investigated for the BCN nanofilms with various carbon compositions. We found that no increase of the turn-on electric field of the electron emission occurs for the BCN film with carbon composition lower than 18%. However, in the range of carbon composition higher than 20%, the turn-on electric field increases rapidly with carbon composition. (c) 2005 American Vacuum Society.