CeO2-x films were deposited by sputtering a metal Ce target in a gas mixture of high purity Ar and O2 using a pulsed unbalanced magnetron sputtering system. In this work, cerium oxide thin films were grown onto silicon substrates under different O2 flow rates, which were varied from 20 to 80% of the total flow rate with simultaneous changes in the Ar flow rate. In addition, different growth conditions and the influence of post-deposition rapid thermal annealing (RTA) were performed to tailor the stoichiometry of the cerium oxide films. The microstructure and mechanical properties of the films were characterized using electron probe microanalysis (EPMA), X-ray diffraction, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and nano-indentation. EPMA results revealed that all as-deposited CeO2-x films have an O/Ce ratio about 1.75. When the post-annealing temperature (TPA) for the films was increased from 800 to 1100 °C, a reduction of oxygen in the film was observed, which led to a phase transition from cubic CeO2-x (111) to hexagonal Ce2O3 (002). This phase transition is related to Ce4 + to Ce3 + cation transformation due to the formation of oxygen vacancies. The hardness and elastic modulus of the as-deposited CeO2-x films were 11.7 GPa and 241 GPa, respectively, which were reduced to about 7.5 GPa and 150 GPa, respectively, after annealing at 1100 °C.