Abstract: Flow patterns and their transition characteristics for horizontal oil-water two-phase flow in a stainless steel pipe loop (25.7 mm inner diameter，30 m long)were studied.The viscosity and density of oil were 620 mPa•s and 872.17 kg•m^-3 respectively at 30 ℃, 180 mPa•s and 858.68 kg•m^-3 respectively at 50 ℃.Visual observation,the local sampling method and the analysis of the pressure drop signals were used jointly，and different flow patterns were defined and distinguished.A new classification for oil-water flow patterns was proposed.The flow pattern maps were obtained from the experimental data, and the factors affecting the transition mechanism of different flow regimes were discussed.In addition, some semi-theoretical criteria for the transition between different flow patterns were proposed.Especially, an accurate model was developed to predict the critical conditions for phase inversion.Comparisons of the proposed criteria with other experimental data showed reasonable agreement.