The polytropic process is used to signify the effect of “energy degradation” associated with the equipment losses, which is as an inherent irreversibility in a compression process. The polytropic process is path dependent, which entails the irreversibility associated with the system. The change of gas composition and operating conditions affect the energy degradation. In this paper the polytropic process of real gas is explained and thermodynamics and mathematical model used in Taher-Evans Cubic Polynomial Method ,  is presented. The elegance of Taher-Evans Cubic Polynomial Method is its rapid solution technique and high precision for calculating polytropic efficiency as required for compressor performance testing by the ASME PTC-10 .