Selective catalytic reaction is a very efficient method to reduce NOx emissions from thermal power plants and is widely used in Japan. To develop a higher performance de-NOx system and optimize its maintenance schedule, it is important to understand the NOx reduction mechanism in the honeycomb channel which supports the de-NOx catalysts. In this study, the effects of duct channel flow behavior on the de-NOx, reaction at the catalyst surface were investigated using a direct numerical simulation (DNS). The DNS computations were performed for three inflow conditions, one laminar and two turbulent. The results show that although the flow transitions from turbulent to laminar flow as the flow moves downstream for the turbulent inflow conditions, de-NOx reaction rates for the turbulent inflow conditions are higher than that for the laminar inflow condition even in the downstream region. This is because of the remaining cross-sectional fluid motions caused by the inflow turbulence. As a result, de-NOx efficiencies for the turbulent conditions are higher than that for the laminar case. For both laminar and turbulent inflow conditions, de-NOx reaction is suppressed in the corner regions due to the flow stagnation. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.