Computing or determining the amount of maximum flood debit in watershed is one of the main important parts of hydrological research in river improvement. In this regard it is necessary to be distinguished peak of flood flow and its hydrograph for favorable return period. This needs to have long time statistic and leminograph tools in study research area. In this study with the comparison of available statistic in hydrometric station and by use of observation, Creager and SCS method, to be tried to estimate the amount of design flood in Gharasoo river basin in Kermanshah province. Having regard between hydrology and geomorphology make it possible in some basin that have the same geomorphologic condition with making the correlation between 2 watershed ; estimating the hydrological statistic in the basin that have lack of (incomplete ) statistics. Here by is provided made natural and synthetic hydrographs by use of data morphologic; hydrometric and hyetometer of watershed and analyze of hydrologic; and method of making the synthetic unit hydrograph; compared with natural unit hydrograph (observing method). With regarding to the high accuracy of SCS method which use of different continent; geometric and physiographic parameters; the results of this method offer for design flood. For determining design flood by SCS method is used HEC-HMS software which used the March 1997's data for calibrating the software. The results indicate maximum flood flow amount of 1520 m3/s in Pole Kohneh station. Also the Creager method was used for determining of maximum flood debit in the watershed which have incompleted statistics .maximum results obtained from this method for Do Ab Qazanchy station,is 1530 m3/s .