A multi-region separation model was developed analytically for determining the cyclone efficiency. The multi-region model used the flow field information from recent studies and more accurate position of the interface between the downward and the upward flows. The model also gave equal consideration to the conical section playing in the cyclone separation process by allowing the calculation of the fractional efficiency of any given particle size directly without doing any geometric simplifications. The comparisons with different experimental results have proved that the multi-region model is very useful in predicting the separation efficiency of the gas cyclone. The sensitive analysis shows that the multi-region model responds well to the changes of cyclone geometries. (c) 2006 Elsevier B.V. All rights reserved.