A numerical method is used to investigate a steam reformer. The reactor is assumed as a porous medium, because it is filled with catalysts of a packed-bed type, and a pseudo-homogeneous model is incorporated for a chemical reaction model. The steam reforming (SR) reaction, water-gas shift (WGS) reaction, and direct steam reforming (DSR) reaction are assumed to be dominant reactions in the steam reformer. The difference in temperature between the inside and outside of the reactor is a driving force in heat transfer, and is affected by the amount of heat adsorption by an endothermic reaction. A modified Nusselt number (Nuts) can represent the heat transfer rate of the endothermic reactor, and thus Nu(M) can be used to describe the performance of the steam reformer. The SR reaction rate is sufficiently activated when Nu(M) around the inlet region is greater than 10, and fuel conversion exceeds 0.9 when the difference in Nu(M) value between the inlet area and outlet area is greater than 5. The correlation between fuel conversion and operating conditions has also been studied by using Nu(M). (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.