Abstract: The roughness was simulated by using regular transverse rectangle ribs on the bottom surface of a rectangular channel, and local flow and heat transfer characteristics among ribs along a roughened surface were numerically investigated.The results showed that various flow structures appeared in different rib spacing regions along the flow direction, according to which three flow regimes in the roughness section were generally classified into—entrance, middle and exit zone.In the entrance zone flow structures varied quickly, particularly vortex distributions close to the wall in the rib spacing region appeared quite different from each other.In the middle zone,the flow became relatively stable and the flow structure was a single vortex.Similar to the entrance zone, in the exit zone the flow experienced a significant change of flow structure.Flow resistance exhibited W-shape at the very beginning of the entrance zone, while it evolved into a kind of sine wave at the end of the roughness section.Generally, heat transfer coefficient tended to be high in the middle area of each rib spacing region. It was revealed that different characteristics of flow and heat transfer performance were determined by various flow structures in different rib spacing regions along the roughened surface.