Abstract: The flow and temperature distribution in U-shaped variable cross-section channels with different rib heights was investigated numerically by a new developed three-dimensional Navier-Stokes code with low Reynolds number k-ε turbulence model. The rib pitch is 25 mm and the rib heights are 1 mm,1.5 mm,2 mm respectively. The Reynolds number Re based on the inlet velocity of the cooling air and hydraulic diameter are 7 500,12 500,18 500,25 000. Numerical results reveal that the average Nusselt number of the channel increases with the increase of Reynolds number. The enhanced heat transfer was achieved with the higher rib height e when Re is equal to 7 500 and 12 500. The higher heat transfer characteristics can be reached for the case of e =1.5 mm when Re is equal to 18 500 and 25 000. The effects of rib heights on local heat transfer show different behavior due to the different local Reynolds numbers and centrifugal force. The higher rib height e enhanced the heat transfer of inflow passage. In outflow passage, the higher the rib height, the better heat transfer behavior is produced for Re =7 500 and 12 500, while for Re =18 500 and 25 000 the optimum rib height is 1.5 mm.