An anti reflective layer for Si photodetectors working at various wavelengths is designed based on a SiO2 film containing Si nanocrystals (nc-Si). It is shown that with a proper amount of nc-Si distributed in the oxide and at the right thickness of the film, a single layer of nc-Si/SiO2 film can serve as a perfect anti reflection coating with very low or insignificant light loss. For example, for a 110 nm layer with an nc-Si volume fraction of 34%, the reflectance and the transmittance at the wavelength of 850 nm can reach 0.008 and 99.99%, respectively.