This study was attempted to derivate empirical formulas for the runoff: ratio during ilood season at three watersheds of Dan Yang, Chung Ju, and Yeo Ju are located at upper, middle, and lower portion of Nam Han river basin, respectively. Obtained formulas for flood runoff ratio can be applied as an element for the estimation, peak discharge for the design of various hydraulics structures which can be concidented with meteorological and topographical condition. The obtained through this study were analyzed as follows. 1. It was found that the magnitude of runoff ratio depends on the amount of rainfall for all studying basins. 2. Empirical formulas 'for the runoff' ratio were derivated as 1-, 1- and 1- at Dan Yang, Chung Ju and Yeo Ju watershed, respectively. 3. The magnitude of runoff ratio was appeared in the order of Dan Yang, Chung Ju, and Yeo Ju are located at upper, middle and lower portion of Nam Han river basin, respectively. 4. It was assumed that in general the more it rains, the lesser becomes the ratio of loss rainfall. Especially, the ratio of loss rainfall for Dan Yang, upper portion of river basin was shown as the lowest among three watersheds. Besides, the magnitude of that was appeared in the order of Chung Ju and Yeo Ju watershed located at middle, and lower part of river basin, respectively. 5. Relative and standard errors of runoff ratio calculated by empirical formulas were shown to be within ten percent to the observed runoff ratio in all watersheds. 6. It is urgently essential that the effect of antecedent rainfall have an influence on the next coming flood should be studied in near future.