Crystalline uranium oxide thin films were deposited in an unbalanced magnetron sputtering system by sputtering from a depleted uranium target in an Ar+O-2 mixture using middle frequency pulsed dc magnetron sputtering. The substrate temperature was constantly maintained at 500 degrees C. Different uranium oxide phases (including UO2-x, UO2, U3O7 and U3O8) were obtained by controlling the percentage of the O-2 flow rate to the total gas flow rate (fo(2)) in the chamber. The crystal structure of the films was characterized using X-ray diffraction and the microstructure of the films was studied using transmission electron microscopy and atom probe tomography. When the fo(2) was below 10%, the film contains a mixture of metallic uranium and UO2-x, phases. As the fo(2) was controlled in the range of 10-13%, UO2 films with a (2 2 0) preferential orientation were obtained. The oxide phase rapidly changed to a mixture of U3O7 and U3O8 as the fo(2) was increased to the range of 15-18%. Further increasing the fo(2) to 20% and above, polycrystalline U3O8 thin films with a (0 0 1) preferential orientation were formed. The hardness and Young's modulus of the uranium oxide films were evaluated using nanoindentation. The film containing a single UO2 phase exhibited the maximum hardness of 14.3 GPa and a Young's modulus of 195 GPa. The UO2 thin film also exhibited good thermal stability in that no phase change was observed after annealing at 600 degrees C in vacuum for 104h. Published by Elsevier B.V.