The oil / water two-phase flow inside T-junctions was numerically simulated with a 3-D two-fluid model, and the turbulence was described using the mixture model. Some experiments of oil / water flow inside a single T-junction were conducted in the laboratory. The results show that the separating performance of T-junction largely depends on the inlet volumetric fraction and flow patterns. A reasonable agreement is reached between the numerical simulation and the experiments for both the oil fraction distribution and the separation efficiency.