The rise of bubbles in viscous liquids is not only a very common process in many industrial applications, but also an important fundamental problem in fluid physics. In this study movement of single bubble in both stagnant and turbulent flow has been simulated. The “one-fluid” approach is likely to continue to remain an important strategy to handle multifluid and multiphase problems. In this approach the Navier–Stokes equations allow for arbitrary changes in the material properties of the fluids. While the differential form does not allow discontinuous material properties, numerical methods based on the finite-volume method are equivalent to working with the integral form of the governing equations, where no smoothness assumption is made. The interface capturing, PLIC-VOF method, that is based on one-fluid approach, is used to track the bubble interface. The VOF approach should be used when small-scale processes occurring near an interface separating the fluid particle from the continuous phase play a crucial role. Many commercial CFD codes now include the option of simulating free-surface or multiphase flows using the VOF method. In the current dissertation, this numerical algorithm is used to simulate 3D bubbles rising in viscous liquids with large density and viscosity ratios representative of the common air–water two-phase flow system. The results of present thesis are presented in three sections. First, movement and deformation of single air-bubble rising through stagnant water has been simulated. After comparing the results both qualitatively and quantitatively by experimental result that is present in literatures, averaged error of 1.2% has been achieved in terminal velocity. Second, the flow field in baffled cylindrical tank stirred by a Rushton turbine impeller has been simulated. Impeller rotation is simulated by both the MRF and SG method. The MRF method is one of the main approaches for approximating unsteady flow in steady condition, while in the SG approach, full transient simulations are carried out using the two grid zones. By means of initializing the SG solver with the converged results of MRF method, the pseudo-steady state condition achieved at 4-5 impeller rotation. Despite increasing the resolution in this model has no influence on prediction of averaged velocities, but it leads to better prediction of turbulent kinetic energy (k).Finally, Bubble movement and deformation in validated stirred tank is simulated in three impeller Reynolds numbers. Shape oscillation and path transition of bubbles show that surface tension force has important effect on bubble shape. Key Words: Bubble, CFD, Stirred tank, Stagnant water, Volume of Fluid(VOF), MRF, Sliding Grid.