This paper presents a numerical study of the gas-liquid-solid multiphase flow in hydrocyclones with different dimensions of body construction, which include the lengths of cylindrical and conical parts and cyclone body size. The turbulent flow of gas and liquid is modelled using the Reynolds stress model, the interface between the liquid and air core is modelled using the volume of fluid multiphase model, and 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 proposed model is first validated by the good agreement between the measured and predicted results, and then used to study the effects of cyclone size and length. The results show that the flow fields in the hydrocyclones with different size and length are different, which results in different performance. A smaller cyclone is helpful to higher efficiency. The cylindrical section plays an inessential role in collecting particles. A long conical section can improve the performance of hydrocyclone considerably. (c) 2006 Elsevier Ltd. All rights reserved.