Abstract: Taking No.46 mechanical oil,air and tap water as working fluids,the flow patterns and their transition property of oilgas-water three-phase flow in the horizontal and slightly inclined pipes were experimentally studied by visualization method and flow pattern identification instrument.The oil-gas-water three-phase flow patterns in the horizontal and slightly inclined pipes system were defined and distinguished by the signals of pressures and pressure differential transducer.According to oil and water distribution in pipes and the total characteristics of gas-liquid interface,the patterns of oil-gas-water three-phase flow were classified,and the flow pattern maps were given.A new correlation for phase fraction of oil-water mixtures was proposed on the basis of experimental data.The transition mechanisms between different flow regimes were analyzed by using the flow patterns transition criterions for traditional gas-liquid two-phase flow combined with different viscosity models developed for different flow conditions.The result shows that the distribution of oil and gas changes greatly under action of air.The property of oil-water mixture changes greatly under different flow conditions.