Rare-earth oxides are promising high dielectric constant gate oxides to be used in metal-oxide-semiconductor devices in future. Properties of rare earth oxides are very different from those of the well-reported high dielectric constant materials such as HfO₂. In this article, basic properties of rare-earth oxides are described comparing with La₂O₃ and Y₂O₃. These are typical rare earth oxides with a large and a middle ion radius, respectively. It is shown that interfacial silicate formation is an important character of rare-earth oxides on silicon substrates. To control silicate formation during thermal process, the addition of alumina into the rare-earth oxides is proposed. Physical and electrical properties of yttrium-aluminate films on silicon substrates are reported.