Kangsabati reservoir receives the flow mainly from the river kangsabati & kumari, situated in the extreme western part of West Bengal state in eastern India. It is considered as a drought prone area with irregular rainfall. The reservoir is flashy in nature and experience huge flood with a high peak during heavy storm within a short duration causing the possibility of flood risks. Therefore, it is essential to predict the runoff & flood volumes in the Kangsabati basin. So, there is a requirement to develop the watershed modelling for the said reservoir in HEC-HMS software. The DEM based watershed delineation is the primary prerequisite for hydrologic modelling in HEC-HMS. In this study, ASTER DEM of 30 m resolution has been used to delineate the catchment area of a real system Kangsabati reservoir in ArcGIS 10.4. Different intermediate results were generated during the model run and finally catchment area map has been generated & basic parameters are also defined. The watershed area obtained from this delineation process is 3622 km2 while it was 3625 km2 reported in the department of irrigation, Govt. of West Bengal. This difference in watershed area is due to errors from the estimation using traditional delineation. The different watershed parameters such as area, flow path, drainage pattern, river slope, position of rain gauges etc. generated from delineated watershed shapefile will be used as input in HEC-HMS to develop watershed modelling for investigating the possibility of flood occurrence in the Kangsabati reservoir. It can be useful for further analysis of sedimentation, improved land use practices, watershed management etc. for the Kangsabati reservoir in future.