This paper presents the results of modelling study of Koshi River. The modelling approach is based on twodimensional hydrodynamic model. The simulation is carried out with model software Nays 2DH. The study analyses the inundation parameters, hazard assessment criteria, flood inundation extent delineation and identification of hazardous areas in different discharge scenarios of 25, 50 and 100 years return periods flow. Based on goodnessof-fit tests and fitting parameters, generalized extreme event (GEV) distribution method is adopted for flood frequency analysis. The model is calibrated with measured water surface elevations and simulation results. The root means square errors (RMSE) and correlation R2 between measured values and simulation results are 0.95 m and 0.98 respectively. The study results conclude that within the stretch of around 50 km from Chatara to Koshi barrage the flood will not overtop embankments and left overbank is under low danger zone. However, islands within the embankments namely Shukrabare, Rajabas, Khairatol, Shivchowk and Galphadiya are vulnerable to inundation. The modelling approach proposed in this study is an attractive option for modelling exceptional flood events when limited data and resources are available.