A three-dimensional oil-water turbulent flow and oil separation process in a double-cone liquid-liquid hydrocyclone (LLHC) is numerically simulated using FLUENT software. The Euler-Euler approach and Reynolds-stress model are combined and adopted in this simulation to handle the challenging situation of anisotropic turbulent two-phase flow with a higher volumetric ratio (over 10%) in the dispersed phase. It is visualized well in the simulation how separation, aggregation and shift of oil and water proceed in the LLHC. The oil separation efficiency is determined based on flow field and phase concentration distribution. The simulation is verified by comparing predicted and measured separation efficiency in the LLHC.