Mechanism of flow induced vibration of a circular cylinder with a splitter plate was investigated by flow visualization techenique using the smoke wire method. When the distance between the cylinder and the splitter plate is within some 2.5 diameter of the cylinder, the shear layer of the stationary cylinder reattachs on the plate not to form the reqular vortex. On the other hand, the shear layer of the vibration cylinder is always rolled up reqularly synchrounously with the vibration even when it reattachs on the plate. When the distance is 1.5 diameter, the strong vortex sheds into the wake by the interaction of the rolling up of the shear layer and the edge of the plate, then the strong flow induced vibration is induced.