Experimental investigations on oil-air-water three-phase flow were carried out in two helically coiled tubes with inner diameter of 39 mm and coil diameters of 265 and 522.5 mm, respectively. The purpose of this work is to provide a basis for the invention and development of a new kind of separation technology for gas-oil-water mixtures with low oil fraction beta less than or equal to 30%. The flow patterns of oil-water two-phase flow and oil-air-water three-phase flow were directly observed in test sections made of plexiglass tubes, The flows observed in coiled tubes could be classified into more than four flow patterns and some flow regime maps were generated and delineated for these tubes. The present results were compared with some results in horizontal flow. The phase inversion characteristics in helically coiled tubes were also discussed. The frictional pressure drop of oil-air-water three-phase flow were measured. The effects of how rates and liquid properties on pressure drop were examined. Based on the experimental data and analysis of the flow mechanism the criteria for the flow pattern transition boundaries between two different flow patterns were proposed in terms of dimensionless parameters. Correlations for the predictions of pressure drop were also obtained.