Abstract: The emission characteristic of oxidized and elemental mercury from fly ashes collected from Guangzhou Pearl River Power Plant（1#）and the First（2#）Second（3#）Electric Fields of Guangzhou Petrochemical Plant was investigated during thermal treatment using Ontario Hydro Methods（OHM）. Under different calcination temperature（200 ℃，400 ℃，600 ℃，800 ℃ and 1000 ℃），and different calcination time（30 min，60 min，90 min and 160 min），the emission amount of oxidized and elemental mercury from the fly ashes was determined subsequently. Results showed that the emission amount ranged from 1.955 ng to 669.87 ng for oxidized mercury and from 14.53 ng to 3501.15 ng for elemental mercury per gram fly ash during thermal treatment. When the fly ash was calcined up to 60 min，all the mercury existing in both forms was mostly emitted. The emission reached to the maximum at calcination temperature of 600 ℃（except for 3#）for oxidized mercury，and at 400 ℃ for elemental mercury. The emission amount of mercury was proportional to the unburned carbon content in fly ashes.