Cr-doped Ga2O3(Ga2O3∶Cr) thin films were deposited on sapphire (α-Al2O3) substrates by dual-target RF magnetron co-sputtering at different oxygen flow rates. Structures and optical properties of the deposited Ga2O3∶Cr thin films were investigated in detail before and after annealing at 900 ℃. The results clearly reveal that the deposited Ga2O3∶Cr thin films are amorphous, and their luminescence mainly appear in the blue-green band. The structures transform from amorphous into polycrystalline after 900 ℃ annealing, and near-infrared luminescence band originated from the doping Cr3+ is well observed. Both the crystalline quality and near-infrared luminescence intensity of the annealed films show strong dependence on oxygen flow rates, while their optical band gaps keep almost unchanged. Moreover, in the range of oxygen flow rates, it has been demonstrated that the Ga2O3∶Cr thin film with the strongest near-infrared luminescence intensity could be obtained under the oxygen flow rate of 4 mL/min. In such case, the obtained film has the better crystalline quality and the amount of Cr3+ substituted Ga3+ is relatively large. This findings could be helpful for preparing Ga2O3∶Cr thin films with better quality.