This study is devoted to the natural vortex length which is one of the most important parameters for the performance of a reversed flow cyclone separator. A mathematical model was developed to estimate the natural vortex length and was applied to a newly designed cyclone which has a cylindrical body with sudden enlargement instead of a conical part. Special attention was given to the effects of inlet gas velocity, diameter and insertion length of the vortex finder and length of the frictional surface. A number of tests were also performed and the experimental data were compared with the model results. Experimental and model results show that, compared to the influence of the other parameters, the height of the frictional surface and inlet velocity affect the natural vortex length considerably. It was also observed that the vortex length increases up to a certain point with increasing the diameter of the vortex finder and then decreases after that. The results also showed that the present model is more successful in predicting the vortex length than the other models reported in the literature. (C) 2012 Elsevier B.V. All rights reserved.