Cubic　boron　nitride　thin　films　were　deposited　on　silicon　(100)　substrates　by　sputtering.　The　films　were　characterized　by　Fourier　transform　infrared　(FTIR)　spectroscopy.　The　reflectance　R(A)　of　the　films　was　obtained　as　a　function　of　incident　photon　wavelengths　and　the　thickness　of　the　films　was　measured　by　Alpha-step.　Using　Kramers-Kronig　transform　and　the　reflectance　spectrum　R(A),　we　calculated　the　absorption　coefficient.　The　optical　band　gap　was　found　to　be　5.38　eV　for　the　films　containing　55.4%　of　cubic　phase.