The approach coupled with computation fluid dynamics (CFD) and complex chemical kinetic computation to predict the oxidation of the elemental mercury in flue gas was discussed in this paper. According to the oxidation mechanism of the elemental mercury, the reactions which were in close relationship with mercury oxidation were determined by the sensitivity analysis method. The mercury oxidation process was simulated under the atmospheric pressure condition with different flue-gas compositions. The three-dimensional concentration distribution of mercury within the cylindrical flue duct and the impact of the temperature, concentration of Cl2, HCl, NO, and O2 on the mercury oxidation were also obtained. The simulation results were compared with the experimental results of Mamani-Paco and Widmer. The results show that coupling computation solves the problem of the combination of the CFD with the complex kinetic mechanism. The promotion effect of Cl2 on the oxidation of elemental mercury is much better than that of HCl. The temperature window ranged from 950 to 1,150 K for the higher oxidizing rate of the elemental mercury was determined. The slight enhancement of NO on mercury oxidization was observed which was controlled by the competition between two reaction pathways. O2 weakly promotes homogeneous Hg oxidation, especially under the condition of high temperature. 1.1, 2.6 and 3.1 % of mercury was oxidized in the presence of 0, 4 and 16 % O2 at 600 K, respectively. However, 12.5, 22.5 and 26.0 % of Hg oxidation has been obtained at 1,200 K.