Abstract: In order to explore the effect of lower size limit of feed slime on operating performance of heavy medium cyclone, experiments are made using a φ200 mm H. M. cyclone with its feed having a varied lower size limits of 0, 0.045 and 0.074 mm. Test result indicates that in such cases the corresponding ecart probable Ep values are respectively 0.115, 0.090 and 0.082 g/cm³, and the Ep value tends to lower down with the uplift of the lower size limit of feed, which is beneficial to the accurate separation of slime.