Abstract: Churn flow and annular flow are common flow patterns encountered in gas-liquid systems through conduits.In this paper,the transition from churn flow to annular flow for upward gas-liquid in vertical concentric annulus is studied and a transition model is developed by considering the structural feature of the concentric annulus and the flow characteristics for churn and annular flows in the concentric annulus.The present model is compared with the experimental data from this paper and other authors along with existing models.It is in better agreement with experimental data than existing models.