Objective to investigate the dependence of nitrogen oxide removal effect on a plasma reactor length, three groups of different length experiments were conducted using a concentric cylinder double dielectric-barrier discharge reactor. Experimental results show that changing the length does not obviously affect the electrical power consumption with a given level of nitrogen oxide removal efficiency. An excessively short reactor is inadvisable because it must dramatically elevate the peak voltage when operating at a relatively high discharge power. A significantly high peak voltage indirectly leads to a rise in the electric energy consumption. An excessively long reactor is also inadvisable because it results in material waste and increasing the flow loss. An ideal reactor needs the length to match the demand of discharge power.