Abstract: The Ontario Hydro Method (OHM) was used to sample and analyze the mercury concentration in flue gas before and after ESP and WFGD in a 300 MW power plant. Mercury content in coal, bottom slag, fly ash of ESP, adsorbent (limestone) and desulfurization product (gypsum) was detected by DMA80. Mercury mass balance was calculated based on the online measurements through the boiler system. Factors affecting the distribution, transformation and removal of mercury in flue gas were discussed. The results show that the gaseous mercury (Hg⁰ and Hg²⁺) in flue gas accounts for about 95% of total mercury, while mercury in the bottom ash can be neglected. More than 95% of Hg^p and a little gaseous phase mercury (Hg⁰ and Hg²⁺)were removed by ESP. The efficiencies to remove total mercury by ESP range from 12.77% to 17.38%. A removal efficiency for Hg²⁺(g) reaches up to 79.93%~90.53% by WFGD, however, the content of Hg⁰ after WFGD increases because part of oxidized mercury is reduced to elemental mercury during WFGD. The efficiencies to remove total mercury by WFGD range from 9.68% to 29.36%. ESP and WFGD can remove all of the Hg^p and a majority of Hg²⁺ with a total mercury removal efficiency of 25.38%~38.38%. In general, the demercuration in the conventional devices of ESP and WFGD is not high, which perhaps is owing to the lower concentration of Cl in feed coal.