The field-emission (FE) characteristics of boron nitride (13N) nanofilms (8-10nm) synthesized on n-type gallium nitride (GaN) substrates are investigated. It is demonstrated that GaN is superior to Si mainly due to its lower electron affinity than Si. BN nanofilms on GaN substrates are proved to significantly improve FE in contrast to BN thick films on GaN substrates. Surface roughening is found to be applicable for nanofilms to further improve FE. An optimum turn-on electric field as low as 3.6 V/mum is obtained from the nanofilm on a roughened surface.