Flow estimation at a point in a river is vital for several hydrologic applications, including flood forecasting, water resource management, and development applications.Estimating the Rainfall-Runoff condition of Rib watershed, which helps toforecast storm events for proper management of water resources. This study presents the result of watershed scale rainfall-runoff modeling on the gauged part of Rib watershed using the hydrologic model HEC-HMS in a GIS environment. The model was used to transform the excess precipitation (after the losses have been subtracted) into surface runoff or direct runoff data, together with the GIS extension HEC-GeoHMS precipitation excess to overland flow and channel runoff. Rainfall data used from nine meteorological gauged stations. The simulation was done for 5 years. The predicted Hydrograph was calibrated against the observed data, and the model parameters were optimized for good simulation. The smaller discharges followed the observed trend,but the actual peak value was far from. The value of simulated peak discharge was found to be 578.9 /s while the observed discharge was 256.9 /s, which has adifference of 322 /s. The model framework developed in the study considered the spatial variation in the runoff response of the watershed by using rainfall data from a number of rain gage stations located in different parts of the basin. For modeling flows in over banks, topographic data should be of high resolution so that the topography of the floodplains could be properly represented and flow data of long -time duration is necessary for the calibration and validation of Hydrologic model. Key Words: HEC-HMS, Rainfall - Runoff, Peak Discharge, Rib