This paper presents a hydrologic method of probabilistic design flood calculation for ungauged small river basins. It is based on the study and analysis of the physiographic characteristics of the river basin for which stream flow records may not be available. Rainfall data is used at nearby station which has the rainfall intensity-duration-frequency relations. Musim cheon, second tributary of the Guem river, is selected for the sample study. Design floods for the stream reaches are computed by the Rational formula, the runoff coefficients being determined with the physiographic data such as soil type, land use and vepetal covers. Derived unit hydrograph at conneted main river basin is used to compute the peak flood discharge. Kajiyama formula and modified Kajiyama formula are used to calculated the most probable maximum flood discharge. The result of this study shows that synthesized unit hydrograph method is more accurate and applicable way to com pute design flood for ungauged small river basins.