Abstract Rainfall duration of design storm is one of most important factors of hydrologic design. However the study is rare to investigatethe characteristics of rainfall duration, this make it difficult the efficient design procedure. In Korea, the critical storm duration hasbeen used for calculating peak discharge for design. The critical storm duration is defined as the storm duration which producesthe biggest flood peak discharge in considering various storm durations. However the suitability of this critical storm duration hasnot been proved in actual rainfall runoff relationships. In this study, annual maximum storm events and observed maximum rain-fall-runoff data at dam stations are evaluated to assess the design rainfall duration, and the results are as follows. The most fre-quent rainfall duration is about 24 hours as extreme storm events in Korea. There was no significant trend between basin areas anddesign rainfall durations. The bigger basin area shows wider rainfall durations. The duration of 24 hours is most appropriate forthe fixed design storm duration if the rainfall is uniformly distributed throughout the whole basin area.Key words : design rainfall, duration, annual maximum storm events, 24 hours