The effect of reaction temperature, NSR(Normalized Stoichiometric Ratio), gas residence time, surfactant species on the reduction reaction of NOx with urea solution have been determined in a flow reactor. The maximum conversion of NOx reduction by urea solution appears about 950-1,000 ℃ and increases with increasing the gas residence time. Conversion of NOx reduction increases with increasing NSR up to 1.5 and then remains constant at 950 ℃. The addition of linear alkylbenzen sulfonate(LAS) into urea solution enhances the conversion of NOx reduction at lower temperature below 800 ℃ and the temperature window showing the NOx removal efficeiny above 70% is widen to 800-1,000 ℃. Addition of α-olefin sulfonate and coconut oil also shows the reduction of the optimum temperature and the widening of temperature window for NOx reduction.