Monolith reactors are widely used as catalytic converters in the automobile industry. In these reactors, the geometry of the channels can be accommodated to the requirements of the specific application. Selective reduction of NOx by ammonia which takes place during the flow of the flue gas through a honeycomb reactor was investigated in this work. The reactants pass through the channels of the reactor and diffused into the washcoat of V2O5/TiO2 catalyst. In the present work, one-, two- and three-dimensional models for these reactors were considered and compared. In two- and three-dimensional models, equations of continuity and momentum were solved simultaneously. Using these models, effect of geometry and shape of channels on NOx reduction was investigated. Different channel geometries, including circular, hexagonal, square and triangular, were considered. Also, effects of important operating parameters, such as temperature, porosity of the washcoat layer and its thickness on the conversion in the reactor were investigated. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.