Rajang River Basin (RRB) is the largest river basin in Malaysia, in the central region of Sarawak Malaysia. There are two large dams (Murum Dam and Bakun Dam) in upper RRB and these are built as a cascade. Bakun Dam is located downstream of the Murum Dam and to assess the flood risk to the Bakun Dam, the estimation of frequent peak flood discharge (PFD) is important. Rainfall-runoff routing modelling was undertaken with RunOff Routing on Burroughs (RORB) tool to estimate PFD for 1 in 2 annual exceedance probability (AEP) up to 1 in 100 AEP. RORB tool was used to derive flood hydrograph, and a hydrological model was established for the Bakun catchment. Based on the analysis, for the 1-day storm, the 2-year and 100-year return period design rainfall are 115 mm and 206 mm, respectively. For the 3-day storm, the 2-year and 100-year return period design rainfall are 188 mm and 344 mm, respectively. The peak flood discharge for 1-day storms is higher than the 3-day storms. For the 1-day storm, the 2-year and 100-year return period, peak flood discharges are 3,867 m3/s and 7,043 m3/s, respectively. For the 3-day storm, the 2-year and 100-year return period, peak flood discharges are 3,632 m3/s and 6,722 m3/s, respectively. © 2020 WIT Press.