The decolorization of methylene blue (MB) dye wastewater was performed via atmospheric Ar non-thermal plasma (Ar-NTP). Investigations were carried out into two main aspects of the model; hydrogen peroxide (H2O2) production and the decolorization of MB by Ar-NTP. It was found that H2O2 could be obtained better at a lower pH and with a higher Ar gas flow rate and plasma power. The decolorization rate constant of MB decreased with the increase in the solution pH, and it was found to follow a pseudo-first-order reaction. The optimum experimental conditions in this paper for the decolorization of a 5 ppm initial MB concentration were: 20.35 W plasma power, 4 LPM Ar gas flow rate, and pH 5, together with the addition of 920 ppm Fe2+. The decorization rate constant was 83.7×10-3 min-1, which was 9.73 times greater than that without the addition of Fe2+ under the same experimental conditions.