Results of numerical analysis of unsteady 3D flow in a model of powerful gas turbine section are presented. The section consists of an inlet nozzle cascade, a last turbine stage model and an exhaust diffuser with struts. The effectiveness of URANS numerical simulation of flow and heat transfer in the outlet diffuser of a gas turbine is demonstrated. It is shown that a three-dimensional unsteady field of parameters corresponding to the outlet section of the flow behind the last turbine stage should be set for adequate numerical modeling at the entrance of the diffuser. Particular attention is paid to analysis of the flow vortex structure. The influence of leakage through the rotor tip clearance on the performance of the exhaust diffuser is evaluated. The computational results are compared with the data obtained at a large- scale experimental rig. Imposing the “second-kind” boundary conditions, heat transfer coefficient distributions for the exhaust diffuser and the struts are calсulated.