Abstract: The new method, using PTC of quaternary ammonium salt in dilute sodium hydro- xide solution, was used to purify the m-dinitrobenzene. It was put forward from the nucleophilic reactivity of the NO_2-group of the aromatic ring. The yield of the product was 97.25% and increased 4% more than by Na_2SO_3 method. The quality of the pro duct was found satisfactory and is confirmed both by the gas chromatography analysis and Infra-Red spectrum. o-nitrophenol in the reaction can be recovered. The waste water is light in color and COD_(Cr) values is lowered by ten times.