Flow patterns and mass transfer characteristics in symmetrical two-dimensional wavy-walled channels are investigated at moderate Reynolds numbers : 20–300. We consider two différent wall shapes: sinusoidal wall and arc-shaped wall. Two wall shapes are quite different in the development from laminar to transitional flow. In particular, a new flow structure consisting of a regular three-dimensional flow is observed at a low Reynolds number for the arc-shaped wall. This phenomenon leads to an earlier transition of turbulence as compared with the sinusoidal wall. Mass transfer characteristics of wavy-walled channels differ from those of a straight-walled channel when flow separation occurs. The arc-shaped wall has a larger mass transfer rate than the sinusoidal wall because of an earlier transition of turbulence.