The transition-piece of an aero-derivative gas turbine engine transfers high temperature combusted gases from the gas generator to a power turbine, and it is manufactured from Iron-Base Superalloy A286 in this investigation. Due to the thermal stress levels developed in the casing of the transition-piece after long operating hours, the life expectancy of the transition-piece becomes shorter. The present study aims to investigate the thermal stress field development in the casing of the aero-derivative gas turbine engine transition-piece in relation to the flow and heat transfer situation inside the transition-piece. The typical operating conditions, under which the transition-piece is subjected, are accommodated in the model study to determine the amount of heat transfer from the combusted gases to the casing using CFD FLUENT code. The two-Dimensional CFD model is adopted in the study due to the axisymmetric flow situation in the transition-piece. Since the day to-day operation varies depending on the engine output demand, the model study is carried out under different operating conditions: (i) typical operating condition, (ii) +-10% change in mass flow rate, and (iii) +-10% change in combusted gas temperature. Temperature predictions of the numerical study are used as input to the thermal stress model to predict the thermally induced stress level in the transition-piece outer casing. The thermally induced stresses are computed through Finite Element Method (FEM) using the ANSYS code. Experimental work is carried out to determine the mechanical properties of the As Received (AR) and Heat Treated (HT) A286 alloy through tensile and 3-point bending tests. The A286 alloy was heat treated to resemble the actual operating conditions of the transition-piece. Using the Scanning Electron Microscopy (SEM) and Optical Microscopy (M), the microstucture of the alloy prior and after the heat treatment is examined. The investigation concluded that the A286 alloy Young's Modulus of Elasticity (E) will reduce significantIy due to operation at elevated temperature and that the fixed ends of the transition-pieces are the most likely meas to fail and result in reduction of tlze life expectancy of the transition-piece.