A three-dimensional computational fluid dynamics simulation of a vortex tube has been carried out to realize the effects of operating pressure. The highly rotating flow field structure and its characteristic are simulated and analyzed with respect to various operating inlet pressure ranges. Numerical results of compressible and turbulent flows are derived by using of the standard k-ε turbulence model, where throughout the vortex tube was taken as a computational domain. The main object of the present research is to focus on the importance of identifying the suitable inlet gas pressure corresponds to used vortex tube geometry. Achieving a highly swirling flow and consequently maximum cold temperature difference were the key parameters of judgment. The results revealed that these acceptable conditions of machine performance can be provided when the inlet operating pressure is appropriate both to mechanical structure of machine and physical properties of working fluid. The stagnation point location in the axial distance of vortex tube and Mach number contours in the vortex chamber as additional information are extracted from flow filed; such that interpretation of shock wave formation regions may be accounted as significant features of investigation. Finally, some results of the CFD models are validated by the available experimental data and shown reasonable agreement, and other ones are compared qualitatively.