In the present study, the thermal and flow fields in a rectangular duct with aspect ratio of 1/8 are investigated experimentally using Liquid Crystal Thermography (LCT) and Particle Image Velocimetry (PIV). Circular ribs were attached staggered on the wide walls and the rib height-to-hydraulic diameter ratio was kept as 0.06 with an attack angle of 60°. Both laminar and turbulent cases were tested. The heat transfer distributions between adjacent inclined ribs were reconstructed by evaluation of the acquired liquid crystal images. A significant spanwise variation in heat transfer coefficient on the rib-roughened wall was observed, with high heat transfer coefficient at the upstream end of the rib and low levels at the other end. In the streamwise distribution between two consecutive ribs, the temperature distribution showed a sawtooth fashion at high Reynolds number. The PIV technique was then applied to perform the flow field measurements to catch the flow structures introduced by the ribs. The spanwise profile of the main flow velocity was found altered, and a secondary flow was detected over the cross section. The strong secondary flow resulted in a three-dimensional flow field. The flow separation zone behind each rib was observed at Re=5800, where the flow redevelops after the reattachment point. This flow behavior was responsible for the detailed distribution of the heat transfer coefficient.