Flood is probably the most devastating, widespread, and frequent natural hazard of thernworld that producing many socioeconomic and environmental consequences within thernaffected floodplains. In Ethiopia, there is a lot of areas under flood problem and Ketarrnriver floodplain located at the downstream of Abura gauging stations is among the mostrnfrequently flood-affected area for which flood risk mapping is so important. Thernobjectives of this study were to model rainfall-runoff using HEC-HMS for Ketarrnwatershed and flood risk mapping by using HEC-RAS and HEC-GeoRAS for Ketar riverrnfloodplain, located downstream of Abura gauging station in Ziway-dugda woreda,rnEthiopia. The HEC-HMS software requires daily hydrological data of 16 years Collectedrnfrom a National Meteorological Agency of Ethiopia for rainfall-runoff modeling. Arnnormal-ratio method was used for filling missing values of precipitation data and datarnconsistency was checked bya double mass curve. The initial and constant loss forrnprecipitation loss model, Clark unit hydrograph for excess precipitation transformationrnto direct run-off, monthly constant for base flow modeling, and Muskingum for floodrnrouting modeling were chosen.Gage weight meteorological method was selected tornassign the weighted precipitation to each sub-basins using the Thiessen polygon method.rnAmong the collected 16 years of hydrological data for rainfall-runoff modeling, 11 yearsrn(1988-1998) were used in model-calibration and 5 years (1999-2003) were used inrnmodel validation. The model-performances were evaluated using performancernmeasuring techniques including Nash Sutcliff Efficiency (NSE) and Coefficient ofrnDetermination (R2). NSE during calibration and validation was 0.72 and 0.67rnrespectively whereas R2 during these two processes was 0.87 and 0.81 respectivelyrnFlood frequency analysis was conducted using HEC-HMSâ€™ frequency storm method forrn2, 5, 10, 25, 50,and 100 year return periods. The peak flood for each respective returnrnperiods were (198.5, 224.1, 242.9, 251.8, 267.5, and 283.5 m3/s). Flood risk mappingrnwas modeled for a peak flood of each return period using HEC-RAS, HEC-GeoRAS,andrnArc GIS software; and resulted in the inundated areas of 2.86, 2.91, 2.94, 2.95, 2.98, andrn3.01 km2 respectively.