Abstract: To investigate the mechanism of liquid-liquid separation in a double-cone hydrocyclone and to improve the separation performance of the hydrocyclone, the authors simulate numerically the three-dimensional liquid-liquid turbulent flow and separation process in the hydrocyclone using FLUENT software. The Eulerian model of a multiphase flow and Reynolds-stress turbulence model were both applied to this simulation for general applications, concerning with dispersed phase with volumetric ratio over 10% and anisotropic turbulent flow dominated by swirling in the hydrocyclone. It is shown well in the simulation that how separation, aggregation and shift of the dispersed phase liquid proceeds in a hydrocyclone. The current simulation is verified by comparing predicted and measured separation efficiency of the hydrocyclone.