Abstract: Mini-cyclones are used to separate superfines about 1 μm from industrial dust for dust classification. A 2-D mathematical model is thus developed, by which the dust classification efficiency is analyzed and proved just in relation to Stokes number. Experimental investigation was conducted for 3 types of mini-cyclones at different air flowrates through their inlets. The result showed that a linear relationship is found between the classification efficiency and Stokes number in the Rosin-Rammler coordinates. A new theoretical expression is derived here, by which the calculated value of classification efficiency is closer to the measured value in comparison with other theoretical expressions. So, it is comparatively an accurate calculation method of separation efficiency. When the air flowrate is in the range from 15 to 25 m/s at the inlets of 3 types of mini-cyclones in experiment, the sizing size of superfines for classification can come down to 0.6 μm.