A computational validation study related to aerodynamic loss generation mechanisms is performed in an axial flow turbine nozzle guide vane (NGV). The 91.66 cm diameter axial flow turbine research facility has a stationary nozzle guide vane assembly and a 29 bladed HP turbine rotor. The NGV inlet and exit Reynolds numbers based on midspan axial chord are around 300000 and 900000, respectively. The effect of grid structure on aerodynamic loss generation is investigated. GAMBIT and TGRID combination is used for unstructured grid, whereas GRIDPRO is the structured grid generator. For both cases, y+ values are kept below unity. The finite-volume flow solver ANSYS CFX with SST k-w turbulence model is employed. Experimental flow conditions are imposed at the boundaries. The flow transition effect and the influence of corner fillets at the vane-endwall junction are also studied in this paper. Grid independence study is performed with static pressure coefficient distribution at the midspan of the vane and the total pressure coefficient at the NGV exit. The velocity distributions and the total pressure coefficient at the NGV exit plane are in very good agreement with the experimental data. This validation study shows that the effect of future geometrical modifications on the endwalls and the vane will be predicted reasonably accurately. The current study shows that an accurately measured turbine stage geometry, a properly prepared block structured/body fitted grid, a state of the art transitional flow implementation, and realistic boundary conditions coming from high resolution turbine experiments are all essential ingredients of a successful NGV aerodynamic loss quantification via computations.