Cubic boron nitride (c-BN) films were deposited on Si(O01) substrates in an ion beam assisted deposition (IBAD)system under various conditions, and the growth parameter spaces and optical properties of c-BN films have been investigated systematically. The results indicate that suitable ion bombardment is necessary for the growth of c-BN films, and a well defined parameter space can be established by using the P/a-parameter. The refractive index of BN films keeps a constant of 1.8 for the c-BN content lower than 50%, while for c-BN films with higher cubic phase the refractive index increases with the c-BN content from 1.8 at χc = 50% to 2.1 at χc = 90%.Furthermore, the relationship between n and p for BN films can be described by the Anderzon-Schreiber equation,and the overlap field parameter γ is determined to be 2.05.