Stilling basins are used to dissipate the kinetic energy of the flow. USBR VI stilling basin is one of the oldest energy dissipators designed for dissipating the pipe outlet flow. This basin looks like a box with the intermediate baffle and an endsill. In this paper, the effect of the USBR VI stilling basin geometry on the entropy generation has been studied numerically by solving the RANS equations. In this research, the entropy generation analysis is used for the first time in the USBR VI stilling basin. In this study, the total entropy generation is introduced as an index for designing the USBR VI stilling basin. The effect of the W/D ratio (Basin width to incoming flow depth to the basin) on the flow field, entropy generation rate, and total entropy generation is analyzed. The results showed that variations of the total entropy generation have a good agreement with variations of the total hydraulic head loss along the basin, mean and maximum velocity on the endsill, and shear velocity near the downstream channel bed. The results also demonstrated that a large W/D ratio is beneficial for the best performance and highest energy dissipation.