The goal of this study is to model rainfall-runoff process using HEC-HMS developed by U.S. Army Corps of Engineers for the 10,508 km(2) catchment that has E01A012-Yenicegoruce stream gage at its outlet which is located just at the upstream of the point where Meric and Ergene Rivers meet. This study is conducted as a part of 115Y064 numbered "Development of a geographical information systems based decision-making tool for water quality management of Ergene watershed using pollutant fingerprints" project funded by TUBITAK. First, meteorological parameters such as daily precipitation and temperature, and daily streamflow data that are observed in and around the study catchment are collected. Then land use, hydrologic soil groups and digital elevation data of the catchment are collected and integrated into Geographic Information System (GIS) environment. Digital maps compiled in GIS environment were transferred into WMS for the calculation of basin parameters, and then the hydrological model for the basin is developed in HEC-HMS using these data. The model is calibrated using daily streamflow values of 1997-2002 and validated for 2003-2005 data. The model results obtained at the Yenicegoruce stream gage has Nash-Sutcliffe Efficiency (NSE) values of 0.8 and 0.75 for calibration and validation, respectively. Hydrological models for Hayrabolu, Luleburgaz and Inanli sub-catchments represented by stream gages D01A008, E01A006 and E01A012, respectively are developed and calibrated as well. Model performances are evaluated using statistical measures such as NSE values and correlations.