ZnO and ZnO-SiO2 nano-scale thin films were deposited on glass substrates by the sol-gel method. The ultraviolet(UV)-visible spectrophotometer was used to analyze their optical properties. The UV-visible transmittance spectra of ZnO films showed that when the mole concentration of ZnO precursor decreased from 0.7 to 0.06 mol/L, besides the absorption edge at 380 nm, a new absorption edge around 320 nm appeared. Furthermore the decrease of mole concentration of ZnO precursor improved the transmittance in the range between 380 and 320 nm. However, ZnO-SiO2 nano-scale thin films only have one absorption edge at 310nm. The optical band gap of ZnO was enlarged when deposited in SiO2 matrix.