Numerical simulations of changes in feed medium solids on dense medium cyclone performance were performed using a multi-phase mixture CFD (Computational Fluid Dynamics) model for medium and air-core coupled with Lagrangian model for coal particles for a 350mm DSM cyclone. The turbulence was resolved using Large Eddy Simulation (LES). The mixture model considered the interactions between water and solid phases in terms of hindered settling, lift and Bagnold forces at high feed medium solid loadings. The medium properties were modified by changing the particle size distribution and concentration. Three different medium sizes (ultrafine, superfine and fine) were used in the simulations. The effect of medium stability and rheology on DMC performance is related to feed medium size in terms of density differential and medium segregation. The simulations predicted low Ep (Ecort probability) values with finer medium which gives high separation efficiency on density. A reduction in cyclone efficiency observed for a given feed medium solids distribution at higher feed medium concentrations due to an increase in slurry viscosity.