In order to investigate the structure parameters evolution characteristics during flow regime transition process in slug flow, the vertical upward slug flow experiment in a wide range of liquid superficial velocity (0.03 < jl < 1.6 m/s) were conducted in a tubular test section with the inside diameter of 25.4 mm. Impedance void meters were employed to measure the void fraction of separated two parts corresponding to Taylor bubble and liquid slug. The present research studied the evolution of length ratio and void fraction in slug unit by keeping the liquid superficial velocity constant while increasing gas flow rate. New structure of slug unit in strong relation with transition process was observed. In specific, it was realized that the proportion of such special structure unit played an important role in transition from slug flow to churn-turbulent flow. The existing transition criteria from slug flow to churn-turbulent flow in upward two-phase flow (entrance effects model, flooding model, wake effects model, bubble coalescence model and Helmholtz instability model) were compared with the experimental identified results obtained by a new objective flow regime identification method, ReliefF-FCM algorithm. The results indicate that the transition model based on the wake effects could be the most appropriate choice to describe the mechanism of transition from slug flow to churn-turbulent flow in present experimental conditions.