In this paper, numerical investigation of upward two phase Flow of air-water has been studied. Different conditions of Flow regimes including annular, wispy annular, Slug, churn and bubbly are simulated based on Hewitt and Roberts map, and a good agreement between the experimental data of the map and the numerical simulation has been observed. Accordingly, a proper CFD model in CFD software of Fluent with the required User Defined Function (UDF) has been obtained to simulate two phase Flows of fluids with large density ratio in vertical tubes. The simulation is carried out with the volume of fluid (VOF) method and piecewise interface calculation (PLIC) algorithm for tracking the interface for the annular, wispy annular, churn and Slug Flow regimes and drift flux model for bubbly with proper selection of computational cell and time step sizes. Furthermore, water and air momentum fluxes have been changed and the changes to the Flow patterns are studied.