Flood routing is one of the most important engineering problems for the design of a spillway, and the procedures for the routing should be thoroughly understood by the engineers engaged in the planning of a spillway. There are many methods for the flood routing such as Muskingum, Steinberg, Puis, Holton, Goodrich, Rutter, Graves, Snyder, etc., which are being used in many countries. This article introduces the theory of the modified PuIs Method in detail which is exclusively being used in the Bureau of Reclamation, Department of Interior, U.S.A. Also, this article includes a routing example worked by the writter for the Ee-dong Reservoir of the Ki-ho Irrigation Association. in Kyong-gi Province.