BN films consisting of c-BN and h-BN phases were synthesized using an ion-beam-assisted deposition process. In contrast to conventional observations, the c-BN and h-BN phases did not form separate layers, but were distributed in the form of nano-sized grains throughout the film thickness. No distinctly aligned h-BN layer was observed before the c-BN phase. Such a mixed character of the film was attributed to a localized ion bombardment effect instead of the macro-stress. Possibly because of the presence of scattered h-BN phases, the thin film described here possessed a low hardness of about 20 GPa and a low stress of about 5 GPa, compared with other reported c-BN-containing films.