The interfacial effect on thermal conductivity is studied with Y2O3 thin films deposited on an Al2O3 substrate. Y2O3 thin films with the thickness between 100 and 500 nm are prepared using rf magnetron sputtering and thermal conductivity of the films is measured using the 3 omega method. The strong film thickness-dependent thermal conductivity due to the interfacial thermal resistance is observed. The film thickness-dependent thermal conductivity is explained by an interface thermal resistance between the film and substrate. (C) 2007 Elsevier B.V. All rights reserved.