The non-thermal plasma injection method was used to oxidize elemental mercury (Hg-0) in this study. A mixture of water vapor and oxygen was selected as the discharge gases. Active species generated by a dielectric barrier discharge plasma reactor were introduced into the flue gas duct, where they reacted with Hg-0. Different parameters including active particles, supply voltage, flow rate of injection gas, system temperature, and typical flue gas components were considered. Experimental results indicated that it produced a high yield of oxidative species than using a single discharge gas, and the reason was believed to be the dissociation and excitation of water molecules and oxygen molecules by electron impact. It was assumed that active radicals include O, O-3, water cluster ions (O-2(+)center dot H2O), and center dot OH. Increasing the plasma injection volume led to higher content of reactive species in the system, which promoted Hg0 oxidation. Approximately 98.3% of Hg-0 was oxidized at 4 kV of voltage with 20 mL/min of plasma injection flow rate. When the supply voltage increased, the temperature in the discharge region increased accordingly, which resulted in the decomposition of reactive species, and the Hg0 oxidation process was restrained as a consequence. The existence of NO and SO2 in the system, respectively, indicated negative effects on the Hg-0 oxidation process, which was believed to be the result of the competitive consumption of oxidative radicals, since both the reaction rate coefficients of SO2 with center dot OH and NO with center dot OH are faster than that of Hg0 with center dot OH.