Dam breach analysis and flood inundation mapping is generally used to predict flood on the downstream of the dam. Dam breach analysis is also crucial to predict dam breach parameters, outflow hydrograph and its downstream nature of propagation. Results from dam breach analysis can be used to protect downstream population and it can also be used while designing and implementing future infrastructure. Megech Dam is under construction phase for the purpose to irrigate 20000ha of land at Denbiya Woreda and to supply drinking water to Gondar Town and around satellite kebeles. In the downstream of the dam, there are many people that have settled near to both the right and left direction of the river and many infrastructures: Schools, roads, bridges, clinics, churches, and huge irrigable lands are found. So, if the dam fails, damage will occur in human life and properties that found within the floodplain. This thesis accomplished using the following software: Google earth, Arc-GIS, HECbGeo-HMS, HEC-HMS, HEC-Geo-RAS, and HEC-RAS. Topographic data for the expected flooded area has been obtained from actual field survey data and from Google earth and TIN of the Study Area was created. The modeling process was towards performing unsteady flow calculations in the intent of routing the breach outflow downstream of Megech Dam from the dam up to the downstream boundary which is 40.19km from the dam. The modeling simulation was performed for two scenarios i.e. for sunny day failure and for probable maximum flood (PMF), but the detail analysis and flood mapping is performed using overtopping failure scenario. Inflow hydrograph with a peak of 2897.2m3/s that were routing by using HEC-HMS model was used as an upstream boundary condition and F.R.L (1947.1m) was used as an initial boundary condition for HEC-RAS model and HEC-RAS gives a peak discharge of 94328.57m3/s. Due to this peak flood, 12921.55 hectares of land, 12576 houses, 68625 people will be affected by flood if Megech rock – earth fill embankment dam fail. Finally, for further study it need to consider detail dam breach flood simulation and flood inundation mapping to prepare Emergency Action Plan.