Numerical simulations of cyclones with various geometries and operating conditions were performed to study the natural vortex length. The numerical solutions were carried out using commercial CFD code Fluent 6.1. A prediction model of the natural vortex length was obtained based on response surface methodology by means of the statistical software program (Minitab V14). The results show that inlet velocity, cyclone length and vortex finder insertion deepness also play an important role in influencing the natural vortex length other than the factors mentioned in publications. Compared with some experimental conclusions, the results indicate that present prediction model can estimate the effects of different geometries and operation conditions on the cyclone's performance more acutely than other models. (c) 2005 Elsevier Ltd. All rights reserved.