Kosi river is the most destructive river in India its location is mainly on north Bihar plane created long history of flooding. It causes death of indigenous people and unending immigration of native villagers from their home. It is destroying the productive power of fields by depositing bunch of sediments mostly sand through over bank flooding and inundation. Many of the paleochannels of the Kosi river also get rejuvenated during the monsoon and their flow into the present channel of the Kosi is also obstructed due to the embankments along the left bank. In fact, the embankment along the left bank is too close to the active channel and the river has no place to spread its overbank flows. Even if lot of research work has been done by flood control management of Indian government in the Kosi river basin but not proper solution is discovered till now to protect the recurrent and extensive flooding, expected reason behind that is the very high variability discharge, high sediment delivery from uplifting neighborhood. HEC –RAS based hydraulic modeling for estimating maximum carrying capacity of one paleochannel has been done in this study. Its geomorphologic information like Shape of paleochannel, flow direction identification and data generation is obtained in Arc-GIS platform, and extracted data from ArcGIS about cross section profile, longitudinal profile, and slope value for reach length used in succession to follow the proper methodology in HEC-RAS to get maximum carrying capacity of desired paleochannel. This would expect as 3500 cusec or (ft³/s). It can be use in holding approximate 2.43 percent of 144000 cusec discharges at breach point in year 2008 flooding. The Kosi river main channel flooded water can be delivering into it for safety purpose. Afterward analysis of the maximum capacity of all the paleochanel in Kosi river basin could also be utilize for this purpose to prevent flood in future. This study is performing by using hydraulic model of HEC-RAS with Arc View software to estimate maximum carrying capacity of the concerned paleochannel for the purpose to prevent flood on Kosi river basin in future.