Experimental and theoretical studies have been conducted about the effect of apex cone at the inlet of cyclone dust box on particle classification. It was also examined about fine control of cut size with new type cyclone. By use of the apex cone at the inlet of the dust box, it is possible to decrease the fluid velocity component in the dust box and to reduce the re-entrainment of particles from the dust box. The cut size of the cyclone with the apex cone is smaller than that without the apex cone. The experimental results obtained agreed with numerical simulation. A new type of cyclone with an apex cone which is covered with a apecially shaped ring allowed cut size movement from 2 to 40 gm. Regarding the movement of cut size by the blow-up method, the cut size is related to the degree of incoming fluid volume inside the dust box. From the visual observation, the cut size increases with the decrease of flowing fluid inside the dust box.