Numerical investigation on the motion of solid particles and the “fish-hook” phenomenon has been carried out in an industrial classifying hydrocyclone. 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 results show that the moving orbit of a particle in the upward flow becomes wider as its size decreases due to the effect of drag force, and small particles would re-circuit between the downward and upward flow, resulting in a high separation efficiency and hence the “fish hook” effect. Therefore, the position and size of LZVV (axial velocity = 0) is critical. Moreover, the effects of some parameters on ““fish hook” behaviour, such as structure of vortex finder, inlet velocity, body size and property of slurry, have been also discussed.