The cubic and monoclinic phases of Y2O3 films were prepared on optical grade diamond substrates. The non-stoichiometry degree of the monoclinic Y2O3 film was higher than the cubic Y2O3 film. Both monoclinic and cubic Y2O3 films had an anti-reflection effect on diamond in 8–12 μm LWIR range. The Y2O3 film with a cubic phase had high resistance to abrasion and plastic deformation.