Abstract: In order to define the relation between the separation size and aperture size of coal slime sieve bend, an analysis is made of the working principle and the law governing the motion of particles on the surface of the sieve bend. Then based on the result of analysis and through numerical fitting of the test data, the above-mentioned relationship is investigated. Result of the study shows that the said relation between the two can be expressed by d=k·L, where the value of K is in a range of 0.534~0.732. The expression derived provides a simple and convenient calculation means for optimum selection of sieve bend in coal preparation engineering design, as well as a guarantee for realizing fine design of coal preparation plant.