Abstract: The fresh and deactivated Selective Catalytic Reduction (SCR) catalysts used in a coal fired power plant were studied in a fixed bed reactor. The physical-chemical properties of the catalysts were characterized by means of SEM-EDX, XRF, XPS, N₂ adsorption/desorption, FT-IR, XRD and TG. The results showed that the used catalyst was seriously deactivated. The NO_x removal efficiency and the specific surface area of the used catalyst (35.0%, 1.05 m²/g) were obviously less than those of the fresh catalyst (88.2%, 72.50 m²/g). The V⁵⁺ content in the deactivated catalyst was increased from 17.4% to 32.2% compared with the fresh one, and large quantities of Al₂(SO₄)₃ can be found over the surface of the deactivated catalyst. The results of SEM and XRD showed that the thermal sintering occurred in the deactivated catalyst. Generally, the V₂O₅-WO₃/TiO₂ catalyst deactivation can be interpreted by the valence change of V atoms, thermal sintering and aluminum sulfate formation over catalyst surface.