In order to overcome the flood risk in urbanized regions with high population density, generation of hydrological models is essential. The aim of this study is to generate a hydrological model for the Ayamama Watershed by using HEC-HMS, simulate the flood event occurred in Ayamama River in Istanbul, Turkey on September 9, 2009 and evaluate the modeling performance of the HEC-HMS. For this purpose, the basin area is divided into the subbasins; the sections and parameters of the natural water channel are defined; infiltration is taken into account with Green-Ampt method and Clark Unit-Hydrograph method is used for calculation of the hydrograph over the watershed. The rainfall data measured during the aforementioned flood event is used in simulation. Peak flow of the hydrographs for the subbasins, which are heavily affected by the flood, are obtained and compared with the results of the calibrated hydrological model generated by Rational Method in WMS in a former study. The results obtained by HEC-HMS model are in good agreement with the results obtained by Rational Method in WMS. The strength of HEC-HMS model over Rational Method is emphasized. Moreover, the subbasins located downstream of the Ayamama River are determined as the most critical region.