In this study, a numerical simulation was performed to investigate the entropy generation behavior of the supercritical water flow in a hexagon rod bundle by using various system parameters, i.e., system pressure, heat flux, and mass flow rate. The Speziale-Sarkar-Gatski Reynolds stress model was verified using experimental data and then used to predict the flow and heat transfer of the supercritical water. The influence mechanism of the system parameters on the entropy generation behavior was discussed in detail. It was found that the system pressure has a minor effect on the entropy generation within the heat transfer enhancement region, while the effects of the mass flow rate and heat flux are significant. The entropy generation of the supercritical water in an inner sub-channel of the hexagon rod bundle decreases with the increasing mass flow rate and increases with the increasing heat flux. A non dimensional entropy generation is defined to fully evaluate the comprehensive effects of the mass flow rate and heat flux. The results show that the increase in the mass flow rate has a greater effect on the reduction in non-dimensional entropy generation in the low mass-flow-rate region rather than in the high mass-flow-rate region. Based on the simulation results, the criteria of non-dimensional entropy generation of 0.4 is set up and the relationship between the mass flow rate and heat flux is determined to balance the minimum irreversibility loss and offer parameters value for practical application in the design strategy of a supercritical water-cooled reactor. (C) 2017 Elsevier Ltd. All rights reserved.