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Mercury oxidation by the V_2O_5/TiO_2 based commercial SCR catalyst was investigated. In coal combustion flue gas, ash derived from coal has been gradually accumulated on the honeycomb surface. By an FE-SEM observation on the honeycomb surface with assistance of FIB(Focused Ion Beam)technique, very fine ash particles with several 10 to several 100 nm of diameter were confirmed to form a porous deposition layer on the honeycomb surface. Both NO conversion and Hg^0 oxidation rates are decreased in accordance with the thickness of the porous deposition layer on the catalyst surface. Diffusion limitation of reactants through the porous deposition layer is one of crucial factor which determines the catalyst life time.