Abstract: In this research, three-dimensional incompressible turbulent flow and heat transfer in a circular tube of U-configuration has been investigated numerically. The influences using the U-tube in various downstream lengths and curvature radius ratios on thermal and hydrodynamic fields are presented in details. Two cases for U-tube are tested, the first case is using the tube with different downstream lengths (1, 0.5 and 0.25 m), the second case is using the tube with different curvature radius ratios (1.5, 2 and 2.5).The tube surface is subjected to a constant heat flux and the air is chosen to be the working fluid with turbulent flow under a range of Reynolds number (10000-25000). The turbulent flow and heat transfer is governed by continuity, momentum and energy equations. The effect of turbulence is treated by a k-g turbulent model. ANSYS Fluent code (15.0) based on finite volume method is used to get the numerical results. The obtained results of increasing the downstream length and reducing the curvature radius ratio showed an increasing in both Nusselt number and friction factor as compared with those of straight tube, for all the considered values of Reynolds number. It is discovered that increasing the downstream length by 50% enhance considerably the heat transfer by about 1.7% and reducing the curvature radius ratio by 100% enhance the heat transfer by about 1.5%, due to the strong intensification of the secondary transverse flows. The maximum enhancement efficiency is obtained for the circular cross-section U-tube with the highest downstream length and lowest curvature radius ratio at Re = 20000. Numerical investigations indicated that within the U-bend, secondary flows partially invert temperature profiles resulting in a significant localized decrease in average fluid temperature at the pipe surface. As a result, a significant heat transfer enhancement is observed. The present numerical results are compared with empirical correlations and verified a comparatively good agreement.