We have investigated the oxidation of gas phase elemental mercury using dielectric barrier discharge (DBD). In the DBD process, active species such as O3, OH, O and HO2 are generated by collisions between electrons and gas molecules. Search active species convert elemental mercury into mercury oxide which is deposited into the wall of DBD reactor because of its low vapor pressure. The oxidation efficiency of elemental mercury has been decreased from 60 to 30% by increasing the initial concentration of the elemental mercury from 72 to 655 μg/Nm3. The gas retention time at the DBD reactor has showed the little effect on the oxidation efficiency. The more oxygen concentration has induced the more oxidation of elemental mercury, whereas there has been no appreciable oxidation within pure N2 discharge. It has indicated that oxygen atom and ozone, generated in air condition determine the oxidation of elemental mercury.