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Summary form only given. Perfluorooctane sulfonate (PFOS) is widely used for industrial and commercial products due to its chemical stability and physical characteristics. Biopersistance and unexpected toxicity however, have raised environmental concerns. In this study, PFOS in water was decomposed using direct plasma methods such as DC plasma generated within oxygen bubbles in water and barrier discharge plasma generated on water surface.The reactor for DC plasma consisted of a ceramic plate, copper electrode, grounded wire, and a container filled with 100 mL, 50 mg/L PFOS solution. The ceramic plate with a hole of 0.2 mm in diameter was attached to the bottom of the reactor. Oxygen gas was fed through the hole at a flow rate of 100 seem, forming small gas bubbles. The copper electrode with a center hole of 1 mm was attached under the ceramic plate and connected to a DC high voltage source through a ballast resistor. A grounded wire was put in the solution. Applied DC voltage initiated oxygen plasma within the bubbles. The typical values of applied voltage and discharge current were 1.6 kV and 7 mA, respectively. The reactor for barrier plasma consisted of a ceramic plate, metal plate and grounded plate. The ceramic plate without holes was placed 6 mm above the 100 mL, 50 mg/L PFOS solution. The copper electrode was on the ceramic plate and connected to an AC high voltage source through a current limiting resistor and a charge measurement capacitor. The grounded plate was put in the solution. The reactor was placed in a closed vessel filled different gas species. By applying AC voltage, barrier plasma initiated between the ceramic plate and the solution surface. The typical values of applied voltage and frequency were 6-15 kV peak, 2 kHz, respectively. A decomposition rate of PFOS was evaluated by measuring the concentrations of F-. With the DC plasma, the generation efficiency of F- was 33.7 mg/kWh after 60 minutes operation. With the barrie- plasma, the generation efficiency of F~ was 26.4 mg/kWh with oxygen and 43.4 mg/kWh with argon, respectively after 60 minutes operation. The generation efficiency using the barrier plasma was comparable to that using the DC plasma. The barrier plasma reactor seems to be suited for large capacity decomposition of PFOS solution.