This study simulated flood runoff of the Gamcheon Basin on the Nakdong River and Ohsipcheon Basin in Samcheok, Gangwon-do, where they frequently recently suffered from flash flood damages. First, consolidating radar rainfall data to consider time and spacial variabilities and development situations is necessary for an analysis of the flash floods. However, radar rainfall data are not reliable enough that it is difficult to use them as measurements of actual ground rainfall. Hence, this paper compared and analyzed the single polarization S-band data of the Gangneung meteorological radar and dual polarization S-band data of the Mt. Biseul radar to grasp their level of contribution to the runoff simulation. Second, a distributed hydrological model is necessary to effectively use GIS information and radar rainfall data of the grid format. Hence, in this paper, we used the S-RAT model as a distributed hydrologic model. The results of this study showed that the accuracy of peak discharge and time when using the distributed hydrologic model and radar rainfall data were similar to the observed values.