Boron nitride (BN) thin films with cubic boron nitride (c-BN) phase were prepared on the (100)-oriented surface of n-Si (0.008-0.02 Omegam) by rf magnetron sputtering physical vapour deposition. The c-BN content is determined to be around 50% by using Fourier transform infrared spectroscopy for the BN thin films. The field emission characteristics of BN films were measured in an ultrahigh vacuum system. It is found that the field emission of the BN film with c-BN phase is evidently more excellent than that without c-BN phase. A turn-on field of 5 V/mum and a current of 460 muA/cm2 were obtained for the BN film with c-BN phase. The Fowler-Nordheim plots of emission characteristics of BN films indicate a straight line, which suggests the presence of the FN tunnelling.