The mechanism of particle separation in a small hydrocyclone is analyzed in this study. The separation efficiency of particles is affected by two major effects, the centrifugal and underflow effects. The fundamentals of these effects on the selectivity curve are derived from the theories of hydrodynamics. Three JIS particulate samples are separated in a 10-mm hydrocyclone. The proposed mechanism can be demonstrated by the available data. An increase in split ratio leads to an increase in the separation efficiency only for fine particles; consequently, a decrease in the separation sharpness results in a remarkable fishhook phenomenon. When the feed rate is increased by increasing the pressure drop through the hydrocyclone, the entire partial separation efficiency will be improved, but the separation sharpness and fishhook depth have no substantial varieties. The proposed mechanism can be used for explaining these phenomena, for the selection of optimum operating conditions, and for the hydrocyclone design. INTRODUCTIONA hydrocyclone is an economic centrifugal solid-liquid separator or particle classifier. Since it has many advantages, such as simple design, low capital and maintenance costs, energy savings, low pollutions, high capacity, continuous operation, etc., hydrocyclones have been increasingly used in many mineral and industrial processes. However, to extend the applications of hydrocyclones, there are still many limitations, especially for the cut diameter and the sharpness of separation efficiency. A so-called fishhook phenomenon on separation efficiency frequently occurs when fine particles are separated in a small hydrocyclone. This phenomenon should be well understood in order to improve the separation efficiency of small hydrocyclones.Similar to those operations in air cyclones, the centrifugal effect is one of the most important factors for particle separation. The centrifugal force in a hydrocyclone is