flood is a sudden happening and quick and destructive event that causes death and financial sensible and unsensiable damages in different parts on the world and Iran annually. Control or decreasing these destructive impositions needs precise and accurate studies. So, recognition of the places with runoff generation potential is very important. In current study, two major aims of investigation of the application of distributed ModClark model in flood hydrograph simulation and determination of flood source area in distributed and sub-watershed condition were investigated in the Khanmirza watershed, Chaharmahal-e-Bakhtiari Province. For this reason, at first, inputs of model were extracted by ArcGIS 9. 3 and then model was calibrated and validated. In next step, in order to determine flood source area for cell units and sub-watersheds, by applying "Unit flood Response" method, at first, design rainfall with return periods of 25, 50, and 100 years at the Aloni station were extracted and then influence of each cell and sub-watershed on output hydrograph of the outlet watershed were obtained. The results of model based on comparison between equal-width discharges show that at validation step, the model was simulated the flood hydrograph with high precision with root mean square error, efficiency coefficient, and R2 of 1. 53, 0. 89, and 0. 74, respectively. Also, according to the results of current study, based on the flood volume relative error, peak discharge, base time, and time to peak, ModClark model had lower error in predicting the flood volume and peak discharge. The final results showed that flood source area increases in sub-watersheds from downstream to upstream, while it doesn’ t follow any distribution in cell units.