Experimental studies have been carried out to assess the effect of conical section length on the performance of particle separation using an electrical hydro-cyclone. With the pre-treatment of beads by milling process, the negative zeta potential of particles increases with the increase of particle diameter. Sub-micron particle classification was possible by use of the special electrical hydro-cyclone under electrical potential of less than 150 V and an inlet flow rate of less than 0.3 l/min. Due to the increase of particle residence time in the cyclone, the 50% cut size decreases with the increase of conical section length. The partial separation efficiency increases with the increase of applied potential or the decrease of inlet flow rate. The experimental data agreed with the proposed model assuming constant surface charge density of particles. (C) 2011 Elsevier B.V. All rights reserved.