Y2O3 thin films were in-situ deposited by ion beam sputtering on Si substrate. The influence of the deposition parameters are studied by X-ray diffraction, electrical measurements and high resolution transmission electron microscopy observations. The stress sate of the oxide layers is investigated by the sin(2)psi method as a function of the deposition parameters and the postannealing treatments. Oxygen ion beam assisted deposition process or post-annealing of as-deposited thin films lead to the same relaxation of the internal compressive stress within the oxide layer. An SiO2 layer sandwiched between Si and Y2O3 is always observed and should play a role both in the growing process and electrical properties of the MOS structure based on Y2O3 oxide layer. The results are interpreted in terms of diffusional process in the oxide. which are directly related to the temperature and the oxygen partial pressure during the growth process.