A one-dimensional model based on the conservation equations of mass and momentum together with the reaction kinetics is developed to predict the axial flow structure in a downer reactor at the varying superficial gas velocity (SGV). The predictions show that, compared to the case of constant SGV, the varying SGV can change the axial flow behaviors for both gas and particle phases and new features appear in the axial distribution of flow parameters such as the gas velocity, particle velocity, solids concentration, pressure and pressure gradient. It is revealed by the simulation that the influences of the varying SGV on the products yield are significant and must be considered in the commercial application of the downer reactor. (C) 2003 Elsevier B.V. All rights reserved.