Multi-phase flows are not only part of our natural environment such as rainy or snowy winds, tornadoes, typhoons, air and water pollution, volcanic activities etc., but also are main processes in a variety of conventional and nuclear power plants, combustion engines, propulsion systems, flows inside the human body, oil and gas production and traort, chemical industry, biological industry, process technology in the metallurgical industry or in food production etc. Therefore, the analysis of these flows is very vital. Drift flux model is one of the methods of the two-phase flow analysis and because of its simplicity is very useful and appealing. The basic concept of the drift-flux model is to consider the mixture as a whole, rather than two phases separately. It is clear that the drift-flux model formulation is simpler than the two-fluid model. However, it requires some drastic constitutive assumptions causing some of the important Characteristics of two-phase flow to be lost. This simplicity of the drift-flux model is exactly what makes drift-flux model very useful in many engineering applications. This thesis provides an assessment of seven well-known models and correlations for predicting the dispersed phase velocity. The secondary phase velocity predictions are compared using the mixture model in Fluent software. Models which were investigated in this study are Ishii, Liao, Gomez, Takeuchi, Morooka, Dix and Manninen. Modeling bubbly two-phase flow was performed using the commercial software Fluent 6.3.26 by using the mixture model that results in incorrect estimation of the volume fraction. Due to incorrect prediction of volume fraction by Fluent mixture model, two-phase flow simulation was performed by homogeneous model and terms related to calculation of the relative speed of phases were applied as source terms in the mixture momentum and secondary phase continuity equations in homogeneous model. Consequently, the wrong estimation problem was resolved. Simulations were conducted for upward bubbly flows by using drift flux models in air-water and water-Therminol flows, and the results were compared with experimental data. It is concluded that the Liao model estimates the best results for predicting velocity of secondary phase in air-water flows. Also Manninen model in air-water cases shows different behavior when input velocities of two phases change. Dix model underestimates results in contrary with Morooka which overestimates results for velocity of secondary phase. Gomez and Takeuchi models overestimate the gas velocity. For water-Therminol simulations, it is demonstrated that Manninen, Ishii and Takeuchi models predicted the velocity of dispersed phase very well. Also Dix and Liao models predicted the same trends which were overestimated. It was also seen that the Morooka model overestimates the velocity of dispersed phase which can be as a result of its simplicity. Keywords: Drift Flux Model, Two phase flow, Fluent, mixture model