Experimental and simulation studies have been conducted about the effects of a ring attached to the upper part of the outlet pipe, and a secondary flow injection near the upper plate, on the particle separation performance of the cyclone. From the experimental and the simulation results, a reverse flow region of rotational fluid was found near the outer side of the upper outlet pipe. Attachment of a ring to the upper part of the outlet pipe was quite effective in reducing this reverse flow region, and cut the size of the cyclone changed to the sub-micron region. Optimum values were obtained for the thickness and length of the ring. The use of a ring decreased the descending angle of particle, but the secondary flow injection increased the descending angle. By using both methods, a ring attached at the upper part of outlet pipe combined with a secondary flow injection near the upper plate, a 50% cut size of about 0.6 mu m, was obtained. The results of a three-dimensional numerical simulation of partial separation efficiency and particle descending angle qualitatively agreed with experimental data.