A computational validation study related to aerodynamic loss generation mechanisms is performed in an axial flow turbine. 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. GRIDPRO is used as the structured grid generator. y+values are kept below unity. The finite-volume flow solver ANSYS CFX with SST k-ω turbulence model together with the transitional flow model is employed. Experimental flow conditions are imposed at the boundaries. The computational predictions are compared to experimental data at NGV exit plane and rotor inlet plane. NGV exit plane measurements come from a previous experimental study with a five-hole probe and the data at rotor inlet plane is taken by the current authors using a Kiel probe with 3.175mm head diameter. The comparison of rotor-stator interface models shows that the stage model, which calculates the circumferentially averaged fluxes and uses as the boundary condition at the interface plane, agrees well with the experimental total pressure coefficient data at the NGV exit. The difference between the NGV only simulation and the rotor-stator simulation is emphasized. The effect of rim seal flow on the mainstream aerodynamics is investigated. This validation study shows that the effect of future geometrical modifications on the endwalls and the vane will be predicted reasonably accurately.