Stormwater detention facilities have been used to reduce flood runoff from urbanized areas. They are recognized as means to reduce peak flows of downstream rivers, and it is needed that the planning of them are done on a watershed-wide basis. In this paper a method to estimate the flood control effects of detention facilities on a watershed-wide basis, and the results of its application to Tsurumi river basin are presented. We could estimate the flood control effects of detention facilities by comparing the hydrographs calculated by the present method for Tsurumi river basin with and without detention basins respectively. It is found that there was little effect on flood control of the small scale detention facilities.