Diamond thin films are deposited on mirror-polished Si substrates using microwave plasma chemical vapour deposition (MPCVD), wherein, the substrates were first treated fornucleation by DC bias-enhanced MPCVD, and diamond nucleation density as high as 1010 cm-2 has been achieved. Under the same density of nucleation, the relationships between optical transmittance and deposition parameters such as methane concentration (n), substrate temperature (T) and gas pressure (P) are established. The results show that the transmittance strongly depends on the methane concentration and substrate temperature, but hardly depends on the gas pressure. With the parameters of n = 0.7 %, T = 840℃and P =20 Torr, a highest transmittance at visible light (λ= 600 nm) reaches to 64.5 %, which is very close to the theoretical transmission limit of diamond film.
