In recently years, flood damage caused by human induced climate change and land use has gradually increased. In order to cope with this flood damage from climate change, it is needed to assess the improper use method of estimate flood amount and the safety of previously constructed hydraulic structures. In this study, it was analyzed the estimated design flood from Thiessen method on a small or ungaged basin and how the selection or the usage of Thiessen method affects estimating the design flood. Furthermore, we widely searched existing cases whose design flood were estimated with Thiessen method. Among those cases, the cases was selected that might be problematic on the watershed division using the Thiessen method. In order to assess the effect of Thiessen method in estimating design flood, and select the standard process suggested by the national flood design manual as follows: (1) selecting a weather station that has long record length and is close to an interested watershed; (2) recalculating the design flood with the standard process. The results indicate that the estimated design flood from the standard process is different from the flood with the Thiessen method. This difference results from the selection of the inappropriate use of multiple weather stations following to the theory of the Thiessen method. Finally, the current study advocates the standard design flood estimation process using a single station is better than the inappropriate use of Thiessen method. In addition, when selecting a single weather station, we prioritize the station with long record length and high credibility, which is generally run by Korea Meteorological Administration(KMA). In addition, Nam River dam was selected in South Korea for assessing the safety of flood from climate change. In order to assess the safety of flood, floods in 5 different cases were estimated as follows: (1) stationary method with observed rainfall data; (2) stationary method with observed rainfall data and RCP data; (3) non-stationary method with observed rainfall data; (4) stationary method with observed flood data; (5) Probable Maximum Flood(PMF). The results indicate that the design flood of Nam River dam was underestimated and the climate should be further considered.