The multiphase flow in a hydrocyclone is studied using computational fluid dynamics (CFD). The Reynolds stress model (RSM) is used to describe the turbulent characteristics of the gas-liquid flow. The motions of particles are simulated by the Lagrangian particle tracking model (LPT). By the numerical simulation, the forces on particles are obtained and their spatial and statistical distributions are analyzed. The separation mechanisms of different sized particles are proposed based on the analysis. The results show that for small particles the main force is the fluid drag force. As the particle size increases, the fluid drag force decreases exponentially, while the effects of the centrifugal force and pressure gradient force are enhanced. The fluid drag forces in the axial and tangential directions are both rather random, while with the increase of particle size the randomness of the radial force gradually decreases. The particle motion is governed by the inward pressure gradient force, the outward centrifugal force and the fluid drag force with strong randomness. For different sized particles, the value of the outward centrifugal force is essentially about 2.5 times of the inward pressure gradient force, so that large particles get into the downstream and are collected in the spigot. As particle size decreases, the magnitude of the fluid drag force increases. In addition, the direction of the fluid drag force becomes more random, though it is generally inward. Under the inward drag force, a portion of particles are pushed to the central axis and escape through upstream. When the particle size is smaller than a certain value, the fluid drag force is significantly greater than the centrifugal force and pressure gradient force. In such case, the movements of particles are largely random, and the particles in different directions are uniformly distributed. (C) 2017 Elsevier B.V. All rights reserved.