Abstract: The separating and flow behaviors of gravity oil-water separator were studied with an oil-water separation simulating system,taking white oil and water as work fluid.The separating efficiencies of six sampling outlets,oil outlet and water outlet were also analyzed.The results show that the maximum separating efficiency of oil pad occurs when oil-water interface is maintained at a certain level,which is dependent to oil viscosity.Water cut in oil outlet decreased with increasing water cut of inlet for the same oil pad thickness.Separating efficiency of water pad was independent to oil content of inlet.There was serious vertex field at the bottom of a separator without internals installed,and its hydraulic behavior was in bad order,which affects oil-water separation.Flow regulating and coalescing internals should be installed in order to improve the oil-water separating efficiency.