Flooding is the most common environmental hazard worldwide. Almost every year, Addis Ababa is affected by floods although their nature and impacts are different from space to space and hydrological nature. This study aims to develop hydrologic and hydraulic models for flood-affected areas of Addis Ababa using modeling tools from Hydrologic Engineering Centre‟s (i.e. HEC-HMS and HEC-RAS). The big Akaki River basin is considered in the study to analyze the effects of rainfall on surface runoff and peak discharges in the downstream flood plain areas. Additionally, the effect of discharge released from „Lega Dadi and Dire dams on the peak flow is analyzed and ultimately produce flood inundation levels to assess the flood map in the study areas. The hydrological and hydraulic modeling is done by dividing the watershed into different sub- catchments. To compute infiltration loss by the SCS CN method which convert excess rainfall to runoff model using Clark unit hydrograph, and channel flow routing accomplished by Muskingum routing method of HEC-HMS model. To examine the accuracy of the simulation, model calibration and validation is performed. Furthermore, the hydraulic model using HEC- RAS is conducted to estimate the flood map of big Akaki river with related flood runoffs within the respective return periods of 25, 50, 100, and 1000 years were analyzed. The output result from HEC-HMS used in the flood mapping of Big Akaki river for the return period of 25, 50, 100, and 1000 year‟s events were determined. Without considering peak discharges from both „Lega Dadi and Dire dams, the values were obtained to be 277.459m3/s, 362.207 m3/s, 462.898 m3/s and 946.851 m3/s respectively. Taking the discharge released from both dams into account, the corresponding peak discharges are estimated to be 354.05 m3/s, 429.15 m3/s, 516.97 m3/s, and 963.10m3/s. By considering both dam flow release, the maximum channel flood depths found in the middle big Akaki river is 8.8m, 9.3m 10.1m, 15.5m for the mentioned return periods and the corresponding areas of flooding are 0.0578km2, 0.0641km2, 0.0698km2, 0.100km2 respectively. Apart from some extreme events, the magnitude flood-prone area for the case of flood released from both dam simultaneously and separately at different time intervals on the downstream flood plain is identified. The flooding impact inundates large downstream flood plain region during high flood level. Furthermore, numerous people living in Akaki River watershed along the river as well as large investment area, agricultural land, and downstream plains are predominately prone to high flood.