Experimental studies were conducted on the classification of flyash particles by use of a modified cyclone with movable 50% cut size on a bench scale. The 50% cut size of cyclone changed from 1 to 10 mu m by the change of apex cone height and blow-up flow rate. When the height of the apex cone was higher at a constant blow-up flow rate, the 50% cut size increased. The reason for this was due to the fact that a decrease in the effective cross-sectional area near the inlet of the dust box gave rise to the increased average upward fluid velocity. In the case of blow-down, two kinds of different-sized fine particles were obtained simultaneously. When the blow-down flow rate was 5%, collection efficiencies of 4 and 28% were obtained with mean particle diameters 2 and 6 mu m respectively. When both the blow-down method and a movable guide plate at the cyclone inlet were used, particles of average diameter of 1.5 mu m were collected. Although the cyclone diameter was 400 mm, elaborate classification of flyash particles was achievable.