This paper presents the numerical simulation of the flow patterns within three cyclones of different cone dimensions. The simulation was performed with commercial CFD package FLUENT V5.5. The accuracy of the simulation was tested by comparison with the available experimental data. Simulation results show that the cone dimension significantly influences the flow pattern within cyclones. The comparison of the flow patterns within the cyclones of different cone dimensions explained well the different separation behavior of these cyclones. The results suggest that the CFD simulation is capable of predicting changes in flow patterns resulting from cyclone configuration modification.