This program is widely used in many countries, but has not been applied to the small basins in Viet Nam because of data missing. Tong Soong bridge on the National Highway crossing Dinh Lap district, Lang Son province, Viet Nam was selected as a case study. The study shows a great potential of the Win-TR55 method to estimate the hydrograph for small and medium watershed in Viet Nam. | Calculation of designed flood in weak data region in Viet Nam using Win-TR55 model