Abstract: The denitrification behaviors of fresh and used honeycomb SCR catalysts were compared and the deactivation reasons were analyzed by using Fourier transformation infrared spectra (FT-IR), N₂ adsorption/desorption, X-ray diffraction (XRD), and scanning electron microscope (SEM). The results show that the functional groups on the surface of the used catalyst are changed and the specific surface area of the used catalyst decreases seriously. Moreover the TiO₂ crystallite of used catalyst is changed and some nano-particles aggregate on the surface of the used catalyst. The reason of the deactivation of use catalyst is the unexpected high temperature (e.g. t> 650℃). Otherwise, the water soluble ions, arsenic and phosphorus in the deposit on the surface of the used catalyst can also affect the catalytic activity.