Abstract: The irreversibility due to laminar forced convection in the entrance region of a duct was studied by means of numerical simulation.Local entropy generation distributions and total entropy generation were obtained based on the entropy generation equation. The effect of Reynolds number on the irreversibility was studied for constant wall temperature and heat flux.The results showed that the irreversibility in the entrance region was different from that in the fully developed region.Reynolds number had a strong effect on the irreversibility in the entrance region.The entropy generation caused by temperature difference was relatively dominant over that caused by viscous flow.The irreversibility at constant wall temperature was different from that at constant wall heat flux.