Y2O3 thin films were deposited by reactive sputtering of an Y target in an Ar and O-2 gas mixture. Intrinsic stresses and the Ar content in the films were measured by the sine square psi method of x-ray diffraction and wavelength dispersive spectrometer, respectively. At low working pressures the films had high compressive stresses. As working pressure increased, compressive stress was relaxed. Ar content was high in the film that had high compressive stress. After annealing of the films at 700 degrees C, the compressive stress was largely relaxed but the Ar content remained unchanged. These results clearly showed that compressive stress in Y2O3 films was not caused by Ar entrapment as an impurity but by Ar bombardment. Intrinsic stress was almost independent of the O-2/Ar flow ratio. showing that O bombardment was equal to Ar bombardment in affecting the intrinsic stress in Y2O3 films. The independence of intrinsic stress with the O-2/Ar flow ratio was explained by the concept of M(i)(1/2){[M(i) cos theta+/-(M(t)(2) - M(i)(2) sin(2) theta)(1/2)]/(M(i) + M(t))} instead of the M(i)/M(i) ratio, where M(t) is the atomic mass of the target material, M(i) is the atomic mass of the sputtering gas, and theta is the scattering angle.