Over the years, ever more stringent requirements on the pollutant emissions, especially NO X , from combustion systems burning natural gas are introduced by the European Union (EU). Among all NO X reduction methods, the flue gas treatment by plasma is widely applied and could be used for both small scale and domestic combustion systems. However, the removal efficiency depends on concentrations of oxygen, water vapor, traces of hydrocarbons, and nitrogen oxides in flue gas. In order to analyze the application of the NO X reduction for small-scale or domestic combustion systems, experiments of NO X reduction by non-thermal plasma from real flue gases originating from premixed methane combustion at different equivalence ratio (ER) values were performed. It was determined that the residual oxygen in flue gas plays an important role for improvement of NO to NO 2 oxidation efficiency when O 2 concentrations are equal to or higher than 6%. The power consumption for the plasma oxidation constituted approximately 1% of the burner power. In the case of ozone treatment, the addition of O 3 to flue gas showed even more promising results as NO formed during combustion was fully oxidized to NO 2 at all ER values.