Boron nitride (BN) thin films were deposited on single-crystal silicon(100) substrates by reactive rf sputtering technique using a boron metal target. A mixed gas of nitrogen and argon was used for sputtering. Different negative bias voltages ranging from 0 to -300V were applied on the substrates. The characterization of films was carried out by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). FTIR analysis indicates that both sp 2 - and sp 3 -bonded BN exist in the film synthesized at -150V bias voltage. By TEM microstructure analysis, the mixed phases in the same sample was also confirmed.