Y2O3 films have great application potential in high-temperature metal matrix composite and nuclear engineering, used as interface diffusion and reaction barrier coating owing to their excellent thermal and chemical stability, high melting point and extremely negative Gibbs formation energy, and thus their structural and mechanical properties at elevated temperature are especially important. Oxygen vacancies exist commonly in yttrium oxide (Y2O3) thin films and act strongly on the phase structure and properties, but oxygen vacancies dependent phase transition at elevated temperature has not been well explored yet. Y2O3 thin films with different oxygen vacancy concentrations have been achieved by reactive sputtering through varying substrate temperature (T-s), in which oxygen vacancies increase monotonously with increasing T-s. For as-deposited Y2O3 films, oxygen vacancies present at high T-s can promote the nucleation of monoclinic phase, meanwhile, high T-s can induce the instability of monoclinic phase. Thus their competition results in forming mixed phases of cubic and monoclinic at high T-s. During vacuum annealing at 1000 degrees C, a critical oxygen vacancy concentration is observed, below which phase transition from monoclinic to cubic takes place, and above which phase transfer from monoclinic to the oxygen defective phase (ICDD file no. 39-1063), accompanying by stress reversal from compressive to tensile and maintenance of high hardness. (C) 2017 Elsevier B.V. All rights reserved.