This paper deals with the computational predictability of aerodynamic losses in a turbine nozzle guide vane (NGV) flow. The paper shows that three-dimensional (3D) computations of Reynolds-Averaged Navier Stokes (RANS) equations have the ability to adequately represent viscous losses in the presence of laminar flows, transitional regions, and fully turbulent flow areas in the NGV of an high pressure (HP) turbine stage. The Axial Flow Turbine Research Facility (AFTRF) used for the present experimental results has an annular NGV assembly and a 29-bladed HP turbine rotor spinning at 1330 rpm. The NGV inlet and exit Reynolds numbers based on midspan axial chord are around 300,000 and 900,000, respectively. A general purpose finite-volume 3D flow solver with a shear stress transport (SST) k–ω turbulence model is employed. The current computational study benefits from these carefully executed aerodynamic experiments in the NGV of the AFTRF. The 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 effect of grid structure on aerodynamic loss generation is emphasized. The flow transition effect and the influence of corner fillets at the vane–endwall junction are also studied. 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 turbine endwall surfaces will be predicted reasonably accurately. The current study also indicates that an accurately defined turbine stage geometry, a properly prepared block-structured/body-fitted grid, a state-of-the-art transitional flow implementation, inclusion of fillets, and realistic boundary conditions coming from high-resolution turbine experiments are all essential ingredients of a successful turbine NGV aerodynamic loss quantification via computations. This validation study forms the basis for the successful future generation of nonaxisymmetric endwall surface modifications in AFTRF research efforts.