Hydrological extremes such as floods and drought are topics of concern for developing countries because of the damage associated with life and property. To minimize such risk, adequate knowledge about river basins is required, which includes flood inundation mapping. In the present research work, the flood inundation map of the Mahanadi River at the Bamnidhi sub-basin for nine different cross sections has been analyzed. Selection of the basin is based on two criteria least human intervention and travel time of surface runoff should be more than one day. The model is prepared by flow simulation using HECRAS. Digital Elevation Model (DEM) with 30m resolution was referred to prepare the terrain for the study area. Here HEC-GeoRAS extension was used to transfer the data from GIS to HECRAS. The results were compared with the NDWI method. It can be further helpful for hydraulic engineers and other agencies dealing with flood mitigation measures.