In this study, the effect of cone ratio (ratio of the vortex finder diameter to the apex diameter. D-o/D-u) on by-pass was investigated for small diameter hydrocyclones. Experimental studies were carried out in a closed circuit laboratory test rig for a wide range of operational parameters, such as diameter of cyclone, vortex finder and apex, inlet pressure and solids content of feed pulp. In the tests, 25.4 mm and 50.8 mm diameter cyclones were used with different vortex and apex units. The results showed that the cone ratio is an important design parameter for cyclones. (C) 2010 Elsevier B.V. All rights reserved.