In this paper, the Degussa P25 TiO(2) (P-TiO(2)) is modified by the post-treatment with the phosphorous acid, and the resulting samples are also characterized by X-ray Powder Diffraction (XRD), Raman spectra (Raman), Brunauer-Emmett-Teller (BET) surface area analyzer, Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectra (FT-IR), X-ray Photoelectron Spectroscopy (XPS), Ultraviolet-Visible Diffuse Reflectance Spectra (UV-vis DRS) and Surface Photovoltage Spectroscopy (SPS). The effects of surface-modification on the thermal stability and photocatalytic activity of the P-TiO(2) are investigated in detail. The results show that the surface-modification enhances the thermal stability of P-TiO(2), even still with a main anatase phase after thermal treatment at 900 degrees C, which is close related to the inhibition effects of the PO(4)(3-) groups on the surface mass diffusion as well as the directing connections of P-TiO(2) nanoparticles. Interestingly, the modified P-TiO(2) by thermal treatment at 700 and 800 degrees C can exhibit much higher photocatalytic activity than un-modified ones. The reasons for the activity enhancement are involved with the enhanced anatase thermal stability, consequently improving photoinduced charge separation rate, and still keeping large surface area and a certain amount of surface hydroxyl groups.