Experiments are conducted to study the heat transfer and friction in a rectangular duct roughened by arrays of alternate attached and detached ribs. The Reynolds number Re based on duct hydraulic diameter ranges from 12 000 to 70 000, whereas the rib pitch-to-height ratio (P-i/h) varies from 10 to 30. The rib to channel height ratio and the ratio of the rib clearance to height are fixed at h/2B = 0.2 and c/h = 0.5, respectively. Results show that the entrance distance for the composite ribbed duct is longer than that for the fully-attached and detached ribbed ducts. The composition ribbed wall yields the highest fully developed heat transfer coefficient, and moderate pressure-drop penalty among the three types of ribbed walls. Performance evaluation under the constant pumping-power constraint reveals that the composite-ribbed duct performs best of the three ribbed configurations. Semi-empirical correlations for friction and heat transfer in composite-ribbed ducts are developed to account for rib spacing and flow Reynolds number.