Diffusers are extensively used in centrifugal compressors, axial flow compressors, ram jets, combustion chambers, inlet portions of jet engines and etc. A small change in pressure recovery can increases the efficiency significantly. Therefore diffusers are absolutely essential for good turbo machinery performance. The geometric limitations in aircraft applications where the diffusers need to be specially designed so as to achieve maximum pressure recovery and avoiding flow separation. The study behind the investigation of flow separation in a planar diffuser by varying the diffuser taper angle for axisymmetric expansion. Numerical solution of 2D axisymmetric diffuser model is validated for skin friction coefficient and pressure coefficient along upper and bottom wall surfaces with the experimental results of planar diffuser predicted by Vance Dippold and Nicholas J. Georgiadis in NASA research center [2]. Further the diffuser taper angle is varied for other different angles and results shows the effect of flow separation were it is reduces i.e., for what angle and at which angle it is just avoided.