Interactions between feed size distribution and separation performance for hydrocyclones with different vortex finder diameters (D-0) are crucial for hydrocyclone designs but not clearly understood. In this work, the optimum range of D-0 is numerically confirmed for a 50 mm diameter hydrocyclone using computational fluid dynamics. Furthermore, effects of feed size distribution (FSD) on separation performance for hydrocyclones with different D-0 are discussed. For a given FSD, the lower limit of D-0 can be determined from the flow stability characterized by lower turbulence intensity and larger tangential velocity. Below the lower limit, a large proportion of fine particles report to the underflow. The upper limit can be determined from separation performance. Above the upper limit recovery to the underflow for coarse particles decreases drastically, which is caused by an outward shift of the locus of zero vertical velocity (LZVV) and an inward shift of particles equilibrium radius. For a given D-0, FSD has no significant effects on partition curves when D-0 is in the optimum range. When D-0 is above the optimum range, both recovery to underflow for coarse particles and separation sharpness are decreased as the feed median size increases. In summary, when D-0 is in the optimum range, the hydrocyclone can adapt to different FSD to obtain the expected cut size by self-adjusting the LZVV and particles equilibrium radius. This method for determining the optimum range of D-0 is based on flow pattern characteristics and can be used as a universal tool for hydrocyclone design. (C) 2017 Elsevier B.V. All rights reserved.