Abstract: The fly ashes from a 320 MW unit boiler with three excess air conditions were sieved to 4 sizes. The carbon contents of the ashes were measured, and the ashes were heated at two heating modes. The Hg contents of the ashes were analyzed using LUMEX Mercury Analyzer, and the Hg species in the ashes was determined according to the Hg release characteristics at different heating temperatures. The activation energies for Hg release were calculated. The results indicate that the Hg concentration in the ashes increases with the decreasing of ash size. Increasing the excess air leads to the decrease of carbon in ash, while the influence of excess air on the Hg content varies with ash size. Hg compounds in fly ash are mainly HgCl₂ and HgS. The rising excess air results in a decrease of HgCl₂ proportion and an increase of HgS proportion, while the proportion of HgO and HgSO₄ keeps almost constant. Residence time is a key factor to influence the formation of particulate Hg. The increasing of excess air and particle size leads to an increase of activation energy for Hg release.