Film cooling behavior in thin rectangular channel was studied experimentally by using water. The film thickness and the wall temperature distribution were visualized with an ink and a thermochromic liquid crystal sheet, respectively. The wavy temperature distributions on the wall just after injection slit was observed in spite of the two-dimensional film. The empirical film cooling model by Goldstein et al., which was developed based on gas flow, failed to predict the film cooling effectiveness using water. The k-ε low Reynolds number turblent model predicted well the experimental results of the relatively low injection rate but overestimated the film cooling effectiveness at the higher injection rate. The overestimation was considered to be due to the underestimation of fluid mixing near the wall in the model.