The Reynolds Averaged Navier Stokes Equation (RANS) model was used to describe the turbulent flow in a coaxial jet mixer. The effects of turbulent models on the turbulent flow is investigated to help the engineers and researchers in deciding the selection of turbulent model need to be used in order to save the simulation time and to predict the best suitable model to be used. Good agreement of the CFD prediction with the experimental data in certain locations was obtained with species transport and velocity profiled, where dependence of turbulent models and grid sizes were discussed in detail. The results show that, the need of grid study is crucial to obtain reliable results with optimum consumption of computer power. SST and Launder Gibson RSTM models give results superior to the other models, each of which has its own area of capability. Launder Gibson RSTM is good in predicting the flow with recirculation and vortices, while SST is good for the flow with less recirculation and high velocity. The overall results show that RANS model is capable in predicting the area of mixing and the velocity profile correctly in certain locations but not in reproducing the vortices structure in the pipe and nozzle. © (2012) Trans Tech Publications.