The occurrence and structure of longitudinal vortices are examined by a flow visualization technique for various sinusoidal wavy channels. Longitudinal vortices are confirmed to appear in a channel with a narrow spacing in the range of geometrical parameters considered here. The geometrical conditions for the onset of such vortices are shown by a diagram. The longitudinal vortex is a kind of Taylor–Goertler vortex observed in a curved rectangular-section channel. However, the wavy channel is different from the curved rectangular-section channel in the processes of vortex development and destruction. It is found that an unstable mode, in which the vortices formed at the upper and lower walls of the channel have some interaction, leads to vortex destruction and significantly promotes the transition to turbulence.