In order to improve the particle separation performance of a hydro cyclone, experimental and simulation studies have been conducted on the modification of the under flow section. The average particle downward velocity in the upper part of the under flow section increases by use of the inclined ring in the Type A cyclone. Particle separation performance of the Type A cyclone is better than that of the conventional Type S cyclone. The number of re-entrainment particles from the center region to the outlet pipe section in the under flow region decreases by use of the center rod in the Type B cyclone. Below the apex cone region of the Type C cyclone, the magnitude of fluid velocity decreases and an increase of particle separation efficiency is obtained. The highest particle separation efficiency is obtained by the Type D cyclone with the inclined ring, center rod and apex cone in the under flow section. The results of CFD simulation qualitatively agree with the particle trajectory observation experiments. The Type D cyclone, which is newly developed in this research, is expected to exhibit better particle separation performance compared to the conventional cyclone, and is expected to have a wide range of potential uses in various wet particle separation processes. (C) 2015 Elsevier B.V. All rights reserved.