The influence of oxygen diffusion on the residual stress of 500 Å thick tantalum thin films sputtered on Si substrate after annealing temperature from 200 to 400 °C is examined in this article. The intensity of diffraction peaks for tetragonal β-Ta phase decreases when annealing temperatures increase by utilizing glancing incident angle x-ray diffraction measurements, and x-ray photoelectron spectroscopy was performed to understand the Ta2O5 oxidation layer on the surface of Ta films after the annealing processes. Furthermore, simulated x-ray reflectivity (XRR) results show that the thicker Ta2O5 oxidation layer is formed when the annealing temperature increases. The compressive stress is nearly proportional to the thickness of the Ta2O5 oxidation layer after the annealing process because of the incorporation of the oxygen into interstitial sites of β-Ta unit cell, which leads to the 5.4 Å3 volume expansion of the β-Ta unit cell. Besides, Auger electron spectroscopy was also used to analyze the depth profile of oxygen diffusion which has consistent tendency with XRR results. The above results indicate that oxidation of Ta films with annealing processes will increase the compressive stress in textured β-Ta thin films.