Effects of acidic gases (CO2, SO2, NO, and HCl) in coal-fired-plant flue gas on mercury removal by a raw activated carbon (AC) under oxy-fuel atmosphere were studied on a laboratory-scale fixed-bed reactor. The temperature programmed-desorption (TPD) method was used to determine mercury forms in the AC. Some characterization methods (BET, FTIR, XRF, and EDS) are adopted to characterize the physical and chemical properties of the AC. Results show that NO and HCl can strongly promote mercury removal, and the mercury species formed on AC are Hg-2(NO3)(2), HgO, and HgCl2, respectively, which all desorb around 300 degrees C. SO2 is not beneficial for mercury removal because it will change into SO3 to occupy the active sites competing with Hg. In addition, Hg2+ adsorbed on AC would be reduced to He in the presence of SO2. Two different kinds of HgS (black and red) are generated after SO2 is introduced; they desorb at 240 and 340 degrees C, respectively. For the oxy-fuel combustion atmosphere, high concentration of CO2, almost has no effect on mercury removal.