Abstract: Coal, slag, and fly ashes were sampled from a 410t/h utility boiler with the equipment of NID (Novel Integrated Desulfurization) system and mercury concentrations of these samples were determined. OntarioHydro method was applied to determine mercury speciation in flue gas before NID and after ESP. The experimental data indicate that the majority of mercury goes into fly ash. The ratio of mercury quantity in fly ash to total combustion product is about 90%, while that in flue gas is about 10%. The results also show that before NID and after ESP, the gaseous mercury concentration in the flue gas is about 21.3μg/m3~22.4μg/m3 and 1.93μg/m3~3.67μg/m3 respectively, indicating that the NID system has quite high mercury removal efficiency up to 83.6%~90.9%. The percentage of Hg2+ which is the main mercury speciation in flue gas before NID is about 67%. The percentage of Hg2+ in flue gas after ESP is about 71.8%~85.1%, while the content of Hg0 is zero, indicating that some chemical reactions have been happened to Hg0 when it passes by NID system. Hg0 becomes Hg2+ and is then adsorbed and removed.