Nanocrystalline silicon dioxide (SiO2) films for optical applications were deposited on aluminium substrates by reactive midfrenquency magnetron sputtering. The composition of the coating is varied by changing the oxygen gas flow during the depositiongs. The structural properties and the surface morphology of films deposited with various deposition parameters were investigated by x-ray photoelectron spectroscopy and atomic force microscopy. The optical properties were measured and calculated by spectrophotometer and thin-film analyzer. It was found that the composition of silicon dioxide films varies from nearly pure Si, SiO to SiO2, controlled by O-2 flow rate. The reflection index of nanocrystalline SiO2 film-aluminum system accords with the mixture rule. All SiO2 films are transparent and the evolution of the transmittance as a function of the films is investigated, and opportunities for their optimization are described.