Single crystalline β-Ga2O3 thin films were deposited on 5.08nm (2inch) c-plane sapphire substrates by magnetron sputtering and then were annealed. The effects of oxygen partial pressure during sputtering on the structural and optical properties of films were systematically investigated. In this study, a single crystalline β-Ga2O3 thin film with a crystalline size of 138nm, a transparency larger than 80% and a wide bandgap of 5.12eV, has been accomplished by modifying oxygen partial pressure. And a β-Ga2O3 thin film has been improved with a roughness as low as 0.401nm and a refractive index of 1.94 at 800nm. The results show that, reducing oxygen partial pressure during sputtering is helpful to improve crystalline quality and increase the transparency and optical band gap by increasing the numbers of sputtered particles as well as their kinetic energy. However, increasing oxygen partial pressure is beneficial to improve the film surface evenness and density.