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Industrial and aeroderivative gas turbines when used in CHP and CCPP applications typically experience an increased exhaust back pressure due to losses from the downstream balance-of-plant systems. Also, gas turbines for mechanical drive application have a wide operating envelope which leads to a fluctuating back pressure that varies with change in exhaust flows. This increased back pressure on the power turbine results in increased exhaust gas temperatures and aerodynamic loading that can influence the mechanical integrity and life of Power Turbine Exhaust System. This Paper discusses the Impact to Fatigue and Creep life of free power turbine exhaust system subjected to high back pressure applications using Siemens Energy approach. Steady state and transient temperature fields were predicted using finite element method. These predictions were validated using full-scale engine test and are found to correlate well with the test results. Full Scale strain gauge survey of the exhaust hood was undertaken at ambient conditions at various pressure levels to validate the structural boundary conditions of lifing models. Strain Predictions were found in good agreement with measured strain gauge data. Steady State and Transient stress fields have been estimated using validated structural and thermal finite element models. Walker Strain Initiation model [1] is utilized to predict Low Cycle Fatigue Life and Larson Miller Parameter Creep Model has been used to estimate creep damage to the exhaust system. The Life Prediction Study shows that the exhaust system design for high back pressure applications meets the product design standards.