The relationship between flow structure and mass transfer in a wavy channel was investigated in the range from laminar to turbulent flow. Laminar flow has a steady two-dimensional structure, but turbulent flow has an unsteady three-dimensional vortical structure. In particular, the flow field in a large recirculation vortex within the furrow of a wavy wall shows an intermittent reversed flow and a nonuniformity of flow in the spanwise direction for turbulent flow. The flow intermittency is closely related to the mass transfer, and a remarkable increment of mass transfer rate is induced near the flow reattachment point in the large recirculation vortex, which suggests a renewal of the concentration boundary layer because of the flow intermittency.