This paper experimentally investigated the bubbly-intermittent flow (BI) transition in helically coiled tubes (HCT) with coil diameter of 20, 40 and 80 cm. A double sensor conductivity probe and a highspeed camera were used to obtain local phase distribution parameters and flow imagery. Because of the centrifugal force, buoyancy was redirected and enhanced, leading to a highly asymmetry and concentrated bubble distribution. As a result, BI transitions occurred at much lower gas flow rates in HCTs. Observation of the BI transition revealed two mechanisms: bubble accumulation in flow direction due to bubble velocity difference (liquid superficial velocity lower than 1.5 m/s), and bubble accumulation in radial and peripheral direction due to strong buoyancy (liquid superficial velocity higher than 1.5 m/s). The critical void fraction at which BI transition occurred (e.g., alpha = 0.065 for coil diameter of 40 cm) increased as the coil diameter increased, and was much lower than that of straight tubes (alpha = 0.25-0.51). A correlation predicting BI transition was established, covering both straight inclined tubes and HCTs; which met the experimental results well when the coil diameter was larger than 0.2 m. (C) 2017 Elsevier B.V. All rights reserved.