This paper presents a numerical study of the gas-liquid-solid flow in hydrocyclones with different shaped vortex finder. It is done based on the mathematical model we developed recently. In the model, the turbulent flow of gas and liquid is modelled using the Reynolds Stress Model, and the interface between the liquid and air core is modelled using the Volume of Fluid multiphase model. The results are then used in the simulation of particle flow described by the stochastic Lagrangian model. The flow features are examined in terms of flow field, pressure drop, split ratio reported to the underflow, particle trajectories and separation efficiency. The model is validated by the good agreement between the measured and predicted results, and here used to study the effect of vortex finder geometry. The results show that separation efficiency decreases for fine particles but increases for relatively coarse particle as vortex finder length decreases. A thin vortex finder is helpful to high separation efficiency, particularly for coarse particles, but results in an increased pressure drop. In particular, the computational results demonstrate that a short circuit flow exists along the outer wall of vortex finder, resulting in a decrease in separation efficiency. To overcome this problem, a new design is proposed for vortex finder and shown to be able to improve the performance of hydrocyclone considerably. (c) 2007 Elsevier B.V. All rights reserved.