The purpose of this paper is to study the effects of a secondary flow injection method on submicron particle separation in a cyclone. It is found that particle collection efficiency increased with an increased secondary flow rate in the upper cylindrical section of the cyclone. In order to achieve a high particle collection efficiency, the location of the secondary flow injection should be close to the upper plate and a high injection velocity should be used. The use of a newly developed cyclone (type A) resulted in a high particle collection efficiency. Nearly all particles are collected by the secondary injection fluid flow. Three-dimensional simulation results are qualitatively agreed with the experimental data. (C) 2004 Elsevier B.V. All rights reserved.