Open channel expansion is a transition that connects a relatively narrow upstream channel-section to a wide downstream channel-section. Such a transition is an important component of many hydraulic structures. Due to an increase in cross-sectional area, channel expansions cause flowing water to decelerate. Under steady flow conditions, flow deceleration will lead to an increase in water pressure that in turn triggers flow separation and creates turbulent eddy motions. These turbulent eddy motions can exist over a long distance downstream of the transition. They cause undesirable energy losses and sidewall erosion. Computational fluid dynamics (CFD) is used to predict the flow characteristics of open channel expansion. Due to their lower time demand and lower cost, these numerical methods are preferred to experimental methods after they are properly validated. In the present study, the CFD solution is validated by experimental results. A limited number of CFD simulations were completed using the commercial Software FLUENT. In particular, mean velocity distributions for the rectangular open channel transitions were used for model validation. The two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations and the two equations RNG k-ε models were used. The validation of the model using test data was reasonable. This paper presents the results of experimental investigations on subcritical flow through gradual expansion in rectangular rigid-bed channels employed by CFD for different value of flow. The velocity distributions of flow through the transition models are made, thus, the efficiencies of the transitions evolved by different value of discharge are evaluated.