The supersonic separator is unique technology which gained market acceptance. The usage in subsea systems have been increasing extensively. The conversion of kinetic energy into pressure energy at the diffuser makes this device more special. The pressure recovery technique helps the system to use near the well head where the pressure is drastically reduced to limited range. In this paper the Numerical study of supersonic separator is done using the computational Fluid Dynamics package FLUENT. The simulations are run using three different fluids, air, methane and natural gas given the same boundary conditions and initial physical parameters. The Redlich – Kwong real gas equation and standard k-? turbulence model is employed. The result shows that the shock formed in the nozzle of the supersonic separator is depending on the density of the fluids, the lighter the fluid the closer the shock position to inlet. The Pressure and temperature variation is also high in low density fluids.