The interaction effect between feed size distribution (FSD) and underflow orifice diameter (D-u) is crucial for the adjustment of the hydrocyclone in plant but not clearly understood. This paper presents a numerical study on effects of FSD on separation performances of hydrocyclones at a wide range of D-u. The numerical results show that both small or large D-u will cause numerous particles being misplaced to the overflow or underflow products, resulting in poor separation performance. These results can be well explained using the predicted water flow pattern and particle distribution. Moreover, there are intense interaction effects between the D-u and the FSD. It is pointed out that for an extremely small D-u, the cut size explodes, and the separation efficiency and separation sharpness deteriorate rapidly with the increase of feed median size (d(0.5)). These behaviors are mainly due to the increase in the coarse particle content as 4. 5 increases, resulting in an increase in the extent of particle accumulation near the spigot. Such accumulation causes a decrease in the tangential velocity and a break in the air core, which in turn leads to a decrease in the flow field stability. Thereby, regular distribution of the particle equilibrium radius along the radial direction is destroyed. As D-u increases, the separation performance of the hydrocyclone is no longer affected by fluctuations in the FSD. Based on results from this study, it is recommended a moderate increase in D-u is competent to mitigate the adverse effects caused by fluctuations in the FSD, to achieve the expected cut size, and relatively high separation precise for particles. (C) 2019 Elsevier B.V. All rights reserved.