Selective non-catalytic reduction (SNCR) of NOX using activated ammonia generated by pulsed plasma has been developed to expand the narrow temperature window for deNOX. The temperature window enlargement of 150 °C was achieved at the lower boundary of the temperature window by activated ammonia injection. Molecular hydrogen in activated ammonia played a key role in the enlargement of the temperature window. The purpose of the present study is to investigate optimum conditions for the low temperature SNCR. DeNOX, which were examined by using an NH3/H2 gas mixture at the temperature ranges of 650-750 °C. The optimum H2/NH3 molar ratio changed by the reaction temperature. The optimum value of H2/NH3 molar ratio was decreased with an increase in the reaction temperature; the ranges of optimum H2/NH3 ratios at NH3/NO = 1.0 were 0.5−0.2 at the temperature range of 650-750 °C. It found that injected hydrogen contributed production of OH radical, which was available for deNOX reactions at the low temperatures. The deNOX level was approximately 80% at NH3/NO = 1.0 under the optimum H2/NH3 ratio at the temperature ranges of 650-750 °C.