Thermal-hydraulic characteristics in a spacer-ribbed annular fuel channel for high-temperature gas-cooled reactors were analyzed numerically by three-dimensional computations under a fully developed turbulent flow. The two-equation k-є turbulence model was applied in the present turbulent analysis, and the turbulence model constants for eddy viscosity and the turbulent Prandtl number were improved from the previous standard values to increase the accuracy of numerical simulations. Consequently, heat transfer coefficients and friction factors in the spacer-ribbed fuel channel were predicted with sufficient accuracy in the range of Reynolds number >3000. It was clarified quantitatively that the main mechanism for heat transfer augmentation in the spacer-ribbed fuel channel was a combined effect of the turbulence promoter effect by the spacer rib and the velocity acceleration effect by a reduction in the channel cross section.