Abstract: Based on the result of calculation of the force to which a material particle is subjected in cyclone,an analysis is made of the correlation of cyclone diameter with feed pressure and particle size.Through a comparative test made with 2 stand-alone cyclones,Φ610-GT and SDMC500-1,the float-andsink data of the 2 cyclones are obtained.Analysis of the data obtained indicates that the Φ610 mm cyclone is superior to the SDMC500-1 version in separation of finer coal when viewed from the points of ash reduction result and ecart probable moyen.