Film cooling behavior in thin rectangular channel was experimentally studied by using water. Wavy temperature distributions on the wall just after the injection slit were observed in spite of the two-dimensional film. The low and high temperature regions corresponded to the high and low speed regions near the wall, respectively. The flow structure was similar to the streak flow in the usual turbulent boundary layer. In the usual turbulent boundary layer, the riblets are effective to reduce the inrushes of high speed main flow to the wall region corresponding to the streak flow behavior. So the riblets were used to reduce the inrushes of high speed main flow into the film in thin rectangular channel. By using appropriate riblets, approximately 30% increase of the cooling effectiveness was obtained.