Abstract: The boron nitride thin films with different cubic phase content were deposited on n-type Si(111) and fused silica substrates by radio frequency(RF) sputtering of two-stage deposition process. The films were characterized by Fourier transform infrared (FTIR) spectroscopy. The transmittance T_e(λ) and reflectance R_e(λ) were obtained as a function of incident photo wavelengths and the thickness of films was measured by Alpha-step. The absorption coefficient was calculated from T_e(λ) and R_e(λ). The optical band gap(E_g) of the films was determined by effective medium form of formula containing E_g. The results indicated that the E_g increases with the increase of the content of c-BN. The Eg is almost equal to calculated results obtained from the empirical formula.