Since there were limited reports concerned with the synergic effects of H2O, SO2, and HCI on mercury and arsenic speciation under oxy-fuel combustion, this paper utilized the results of the thermodynamic equilibrium calculation with FactSage 5.2 to predict the speciation of mercury and arsenic under oxy-coal combustion. Results showed that the percentages of HgCl2 and HgS were higher under oxy-coal combustion atmosphere than those under air-coal combustion atmosphere within the entire range of temperature. It also indicated that H2O(g) inhibited the generation of HgCl2 and HgS and that the mole percentage of HgCl2 was increased by 1 or 2 orders of magnitude, with the concentration of HCl increased by 5 times or 10 times under oxy-coal atmosphere. Arsenic, As-2, and AsN are three dominant arsenic species from 900 to 1400 degrees C under both air- and oxy-coal combustion atmosphere. Besides, the effects of H2O(g) on arsenic distribution was related to the H2O(g) concentration in the flue gas. These results are important for mercury and arsenic control during the oxy-fuel combustion process.