Diffusers are important elements in turbomachinary and are commonly used in many other fluid devices to convert kinetic energy into pressure energy. Annular diffusers appear quite commonly in turbomachinery as exducers in turbines, diffusing elements in compressors and interstage duct system in multistage turbines. Annular diffusers present several intersecting features such as the flow separation and reattachment, regions of recirculating flow, strong streamline curvature, adverse pressure gradient and developing boundary layers which are all difficult to calculate. The design and performance of these diffusers are dependent on large number of geometrical and fluid dynamical parameters. In this work, a numerical study is conducted to determine the effect of swirling incompressible/compressible flow on the performance of annular diffusers. Turbulence is being simulated via two different models, the two-equation k-¿ model and the two-equation k-¿ model. The study is performed with emphasis on the performance of these turbulence models on view of the comparisons with the available experimental data. The comparisons indicated that k-¿ model gives better prediction.