The　temperature　dependence　of　the　cubic　phase　content　and　optical　properties　of　Boron　nitrogen　(BN)　thin　films　was　studied　in　this　paper.　The　BN　thin　films　were　deposited　on　fused　silica　and　Si　substrates　by　radio　frequency　bias　magnetron　sputtering.　The　BN　film　properties　before　and　after　annealing　were　characterized　by　Fourier　transform　infrared　spectroscopy,　X-ray　photoelectron　spectroscopy　and　UV-visible　transmittance　and　reflection　spectra.　The　results　indicate　that　annealing　temperature　has　a　significant　effect　on　the　optical　absorption　edge,　optical　absorption　coefficient　alpha,　refractive　index　n,　and　optical　conductivity　sigma　of　BN　films.　The　optical　absorption　edge　shift　to　the　high　energy　with　annealing　temperature　increase　indicates　that　the　film　optical　band　gap　E-g　becomes　large.　The　change　in　the　optical　absorption　properties　results　from　the　stress　relaxation　and　phase　transformation　owing　to　the　high　temperature　annealings.　The　dependence　of　alpha　on　the　photon　energy　is　fitted　by　the　Urbach　tail　model　in　order　to　determine　the　Urbach　energy　E-0.　In　addition,　it　is　found　that　refractive　index　n　exists　clearly　different　dependences　on　temperature　in　visible　and　ultraviolet　regions,　and　the　optical　conductivity　sigma　threshold　moves　to　high　energy　with　increasing　annealing　temperatures　in　the　threshold　region.　(C)　2008　Elsevier　B.V.　All　rights　reserved.