In order to investigate the evolution characteristics of structure parameters during flow regime transition process in gas-liquid slug flow, the vertical upward slug flow in a wide range of liquid superficial velocity (0.03< j(i)<1.6m/s) are studied in a tubular test section with the inside diameter of 25.4mm. Impedance void meters are employed to measure the time series of cross-sectionally averaged void fraction of separated two parts corresponding to Taylor bubble and liquid slug. With increasing flow gas flow rates at constant liquid flow rate, the present research studies the evolution of length and void fraction of each part in slug unit. New structure of turbulent slug unit in strong relation with transition process is observed. It is found that such special structure plays an important role in transition from slug flow to churn-turbulent flow. It is also found that the flooding determines whether the flow regime evolves into churn-turbulent flow or directly into annular flow. By studying the evolution of flow regimes, two structure parameters, void fraction of liquid slug and length ratio of Taylor bubble to liquid slug, can be used as the transition criterion. New criterion for the transition from slug flow to churn-turbulent flow was also proposed based on these two parameters. It is found that the current results agree well with the existing flow regime map for relatively small diameter tubes in literature. (C) 2018 Elsevier Ltd. All rights reserved.