Huge losses in human lives and economic properties can be resulted from dam failure. Therefore, modeling of dam breaching is significance for dam failure damage assessment, risk analysis, disasters control and mitigation. Hence, this research focused on dam breach modeling and downstream flood mapping of Ajimaa dam, Nile basin, Ethiopia. The general information of dam, metrological data, hydrological data and DEM were collected from ANRSWIEBD and NMSA used to model Ajimaa dam breach using HEC-RAS 5.0.3. During the analysis of rainfall-runoff data in HEC-HMS, the performance of the model was checked using Nash-Sutcliffe Efficiency and coefficient of determination (R2) and their value are 0.558 and 0.6326 for calibration and 0.533 and 0.5653 for validation respectively. The breach parameters were estimated using three regression equation of MacDonald and Langridge-Monopolis (1984), Von Thun and Gillette (1990) and Froehlich (2008) both for overtopping and piping mode of failures. HEC-GeoRAS was used to extract geometric information from DEM and then imported in to HEC-RAS to perform 1_D unsteady flow simulation. The simulation results were mapped using GIS extension tool HEC-GeoRAS (RAS Mapping). The result from the three regression equation, the breach width and breach development time was 50m & 2.05hr, 149m & 1.21hr and 81m & 0.89hr respectively for overtopping and 57m & 2hr, 141m & 1.13hr and 68m & 0.96hr respectively for piping failure. From this result, Froehlich (2008) regression equation was selected for estimating breach parameters both for overtopping and piping mode of failure for Ajimaa dam breach. The 1_D unsteady flow simulation result indicated that, the maximum peak discharge for overtopping and piping failure were 17854.62 m3/s and 17241.22 m3/s respectively. This indicated that, the dam break by overtopping mode of failure will develop more risks than piping mode of failure. From RAS Mapping result, the maximum depth, velocity and inundated area was 36.076m, 24.413 m/s and 859.19ha (8.59 Km2). Generally, this research indicated that, Ajimaa dam will fail both by overtopping and piping failure and the incoming flood will affect both human live and economic properties those located on Ajimaa downstream dam. Therefore, the concerned body should guide communities those living and farming at Ajimaa downstream near the river to protect or minimize the possibility of damage.