This study was conducted to derive optimal design floods by Generalized Extreme Value(GEV) and Weibull-3 distributions for the annual maximum series at ten watersheds along Han, Nagdong, Geum, Yeongsan and Seomjin river systems. Adequacy for the analysis of flood data used in this study was established by the tests of Independence. Homogeneity, detection of Outlines, L-moments. Design flood obtained by Methods of Moments and L-Moments using different methods for plotting positions in BEV and Weibull-3 distributions were compared by the Relative Mean Errors(RME) and Root Mean Square Errors(RMSE). The result was found that design floods derived by the L-moments using the other formulas for plotting positions from the viewpoint of Relative Mean Errors and Root Mean Square Errors.