We examine numerically and experimentally self-sustained oscillation and fluid mixing in grooved channels with different groove lengths. The critical Reynolds number for the onset of self-sustained oscillation decreases as the groove length becomes larger, but oscillatory flow is found to arise from the same Tollmien-Schlichting waves triggered by a sheer layer above the groove. Momentum transfer due to the oscillating parts of the flow is analyzed by looking at the oscillatory stress and the production of oscillatory energy. Fluid mixing processes between the channel and groove flows are also explained by a particle advection procedure. The agreement between numerical and experimental results is satisfactory.