In the present study, flow regimes of co-current, air-water two-phase flow in a vertical tube with 70 mm internal diameter were investigated. Simulation accomplished by open source software, OpenFOAM, and One Fluid model has been used to simulate two-phase flow, which in this model, the interface of two-phase flow has been followed by Volume of the Fluid model. Hitherto, most of the researchers conducted experimentally and the researchers in many of numerical studies just investigated the small tubes. The simulation had investigated according to boundary conditions of the vertical tube. Air and water superficial velocities in inlet and pressure in outlet were constant. Moreover, a no-slip condition in the internal tube walls has been considered. The main purpose of this study is to identify the flow regimes based on the superficial velocities of air and water in the inlet. Moreover, the diagrams of density distributions of phases were obtained, with respect to the behavior of each two-phase flow pattern which can be identified. Superficial velocity of air and water were in the range of 0.01-15 m/s and 0.1-1.5 m/s, respectively. By analysis of results, bubbly, slug, churn and annular regimes; furthermore, semi-annular and Cap-Bubbly sub-regimes were observed.