The paper reports the syntheses of c-BN films by the reaction of B2H6 and NH3 in the hydrogen and argon mixture using microwave plasma-assisted chemical vapor deposition (MPCVD). The effects of NH3/B2H6 ratios, hydrogen addition, pulsed-DC substrate-bias voltage and diamond as bufferlayer on the formation of c-BN were investigated. As-grown films are characterized by FTIR, grazing-incidence XRD, and SEM. The NH3/ B2H6 = 3 was selected to obtain a better crystallized BN films with moderate deposition rate. When the hydrogen content was at 85 vol.% in the gas mixture and the substrate bias at −200 V, the maximum c-BN content about 65 % was obtained. However, the film was easily cracked in a few hours. The film deposited at lower substrate biasing of −100 V could preserve for a few days without peeling off, although it had lower c-BN content about 40 %. Nanocrystalline diamond film, deposited from 1 vol.% CH4/Ar plasma, as bufferlayer can promote the heteroepitaxial growth of c-BN, resulting in 80 % c-BN content. The c-BN/diamond bilayer were adherent with a total thickness of 1.2 μm.