Non-thermal plasma can produce reactive species such as O, O3 and OH to induce dye degradation in water. Many chemical reactions are known to occur at the plasma water interface but contributions from each reactive species for the degradation are still not clear. This research investigated the discoloration of methylene blue using a special configured dielectric barrier discharge reactor, where the plasma was injected into the methylene blue solution through micro holes to initiate the oxidation reactions at different pH values of 2.9, 7.1 and 11. The degradation effects from hydroxyls and ozone were determined separately by using radical scavenger Tert-butanol. Results demonstrate that the methylene blue is degraded by both ozone and reactive hydroxyl radicals with the latter contributing up to 30% of the discoloration effect depending on solution pH values. The plasma produced hydroxyl radicals up to 4.6 g/kWh and the decomposition rate was not affected by solution pH values; the corresponding methylene blue degradation is 184 g/kWh.