Ultra-thin yttrium oxide (Y2O3) films (physical thickness of 7 nm) were deposited on p-Si substrates by RF sputtering for MOS applications. The structural properties of the Y2O3 gate dielectrics were studied after RTA from 650 to 850°C. The crystalline phase and chemical bonding state of the films were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. According to the XRD patterns, Y2O3 films remain amorphous after 850°C annealing. Moreover, also confirmed by XPS results, the formation of yttrium silicates (YSiO) was observed after 650°C annealing, and the silicate thickness increases with the annealing temperature. It is suggested that the thickness of the silicate layer YSiO dominates the gate leakage current of the MOS capacitors.