A direct numerical simulation(DNS)of turbulent flow in a wavy channel was carried out using the consistent finite-difference method. The boundary wave in the mainstream direction causes periodic pressure gradient, successive acceleration and deceleration, and flow separation. Tuning the amplitude and period of the wave allows continuous observation of turbulence structure from a simple flow in a plane channel to a complex one. The drag imbalance on flat and wavy wall sides is mainly caused by the pressure drag, while the friction drags are almost even between the two walls. The characteristic of the flow field can be explained by a simple linearized analysis. Good agreement between linearized and DNS results validates the thin-layer approximation. A discrepancy is observed in the limited region where intermittent and local separation takes place even though the mean flow does not separate from the wall.