CFD simulations were performed to study the flow and heat transfer in a U-duct, relevant to internal cooling of the first-stage turbine component in electric-power-generation, gas-turbine engines. Parameters studied include (1) two aspect ratios of the duct cross section, i.e. H/W = 1 and H/W = 0.25; (2) smooth duct and duct lined with pin fins of height H arranged in a staggered fashion; and (3) two rotational speeds: 0 rpm and 3,600 rpm. In all cases, the wall temperature is 1173 K; the coolant temperature at the U-duct inlet is 623 K; and the back pressure at the exit of the U-duct is 25.17 atm. The Reynolds numbers studied are 150,000 for the duct with the 4-to-1 aspect ratio, and 150,000 and 375,000 for the duct with the 1-to-1 aspect ratio. When there is rotation at 3,600 rpm, the rotational numbers corresponding to these Reynolds numbers and duct aspect ratios are 0.592, 1.64, and 4.11, respectively. Result is presented to show the nature of the flow, the temperature distribution, and the surface heat transfer with focus on the flow and heat transfer in the tip-turn region as a function of the parameters investigated. This computational study is based on 3-D steady RANS. The ensemble-averaged continuity, compressible Navier-Stokes, and energy equations were closed by the thermally perfect equation of state with temperature-dependent gas properties and the two-equation realizeable k-ε turbulence model with and without wall functions.