Abstract: To study the heat transfer characteristics in a rectangular duct rib-roughened by periodic alternate scale ribs,six models with different rib scale were studied by employing commercial computational fluid dynamics(CFD)software CFX.The results show that,as fluid passes through each rib,a trumpet-shaped vortex is generated from the upstream vertex of the rib,shaping a region with high value of heat transfer coefficient(HTC).The area of high HTC region is affected by both the fluid flow condition and the location of the adjacent downstream rib.As compared with the duct roughened by unique size ribs,planting a smaller rib in a proper location between each two original ribs can improve heat transfer downstream re-attachment region.This also can decrease flow resistance coefficient in the duct and improve the general heat transfer ability.