Gallium oxide (Ga2O3) thin films are deposited on silicon and quartz glass substrates by reactive DC magnetron sputtering under different oxygen pressure η (η =O2/Ar+O2), and the effect of oxygen pressure on the structure and optical band gap (Eg) is investigated. X-ray diffraction (XRD) and Raman scattering reveal that the products are beta-gallium oxide after heat treatment at 900 ℃, and that the grain size and optical band gap of gallium oxide are increased, the band gap Eg varies from 4.68 to 4.85 eV when tested by a room-temperature ultraviolet-visible (UV-VIS) spectrophotometer, and the (Eg) has also been calculated by using Tauc formula while the oxygen pressure η gradually increases.