Abstract: Fouling is considered as one of the important factors affecting the performance of heat transfer and flow process in heat transfer equipment.The existing studies neglect the simultaneous effect of fouling on heat transfer and flow performance and have not demonstrated the effect of fouling on the variation of irreversibility loss.This paper is based on the first and second laws of thermodynamics and the effect of fouling on the thermodynamic performance of convective heat transfer process through a duct with constant wall temperature is investigated analytically.A criterion evaluating the effect of fouling on the thermodynamic performance of convective heat transfer process--the entropy generation increase rate per unit heat flux is put forward.The effect of Reynolds number (not considering fouling) and dimensionless inlet temperature difference on the entropy generation increase rate per unit heat flux is discussed.The results show that the entropy generation increase rate per unit heat flux increases with the increase of Reynolds number and dimensionless inlet temperature difference, the entropy generation per unit heat flux caused by heat transfer decreases and that caused by viscous flow increases when fouling appears and mass flow rate is constant.The criterion put forward in this paper can not only illustrate the effect of fouling on heat transfer process, but also on flow process.In addition, the entropy generation caused by conduction of fouling plays an important role in total entropy generation of heat transfer process through a duct.The research reported in this paper provides a valid way to analyze quantitatively the effect of fouling on heat transfer and flow process performance.