In this paper an experimental research on the oil-water liquid-liquid two-phase flow patterns and their transitions in both horizontal and vertical helically coiled tubes is presented. The different flow patterns are defined and distinguished from each other according to the relationship between oil and water phase in the tubes. The flow pattern maps and phase inversion points are obtained and compared with the results in horizontal pipes. The effect of the dimensions and different locations of the helically coiled tubes on flow pattern transition and phase inversion point is also examined.