Non-equilibrium pulsed discharge plasma using a gas-liquid slug flow in a glass column as a plasma reactor is developed and applied to methylene blue (MB) dye decolorization in aqueous wastewater. By flowing the solution and gas simultaneously in the slug flow system, the interval of the bubbles could be controlled and adjusted for each flow rate. After the slug flow became steady, an electrical discharge was introduced into the system using an AC power supply with a bipolar pulsed output voltage to generate plasma in the bubbles between the electrodes with a high applied voltage. The decolorization rate of the MB dye was determined by UV-Vis absorption spectroscopy. Based on the experimental results, it was determined that approximately 90% of the MB dye was decolorized for one-time treatment in oxygen and argon in a gaseous phase. This suggests that the oxygen-based reactive species derived from oxygen gas were effective as oxidizing agents, whereas short lifetime OH radicals were mainly produced and decolorized the MB dye in the case of argon and helium gas. [GRAPHICS] .