The objective of this study is to estimate the flood runoff for three guaged stations within Namgang-Dam watershed which are operated by KWATER. For a flood runoff simulation, HEC-HMS was applied and the simulated runoff was compared with observed from 2004 to 2008. The watershed area of Sancheong, Shinan, and Changchon were 693.6 $km^2$, 413.4 $km^2$, and 346.48 $km^2$, respectively. The average runoff ratio of observed runoff for three watersheds were 0.725, 0.418, and 0.586, respectively. The dominant land cover of three watersheds are forest with the value of 71.6 %, 73.1 %, and 82.0 %. Three different cases according to the potential maximum retention of forest areas for calculating the curve number were applied to decrease the error between the simulated and observed. The simulated peak runoff of case 3 which applied the 90 % of potential maximum retention of curve number which is equivalent to AMCI for all the AMCI, AMCII, and AMCIII conditions showed least root mean square error (RMSE). The case 1, which was suggested by previous study, showed high discrepancy between the simulated and observed. Since the forest area consists of more than 70 % for all three watersheds, the application of curve number for forest is critical to improve the estimation errors. Further research is required to estimate the more accurate curve number for forest area.