Unsteady flows in a channel with oblique plates have been numerically investigated. The oblique plates as disturbance promoters are installed at the upper wall. Unsteady characteristics are examined for Re = 350 and the plate angles of a = 60^。- 120^。. The flow fields represent three-dimensional features variously as the plate angle varies. From frequency analysis, it is noted that the disturbed flow by the oblique plates has peculiar unsteady modes. As the flow is more unstable, multiple frequencies are appeared.