Flood is one of the major natural disasters in Iran, which is mostly, affected most parts of the country and caused hazards. Therefore, identification of the area with high potential risk of flood occurrence is the main purpose in order to the flood control and reducing its damages. In this research the role of the flood routing in identification of the area with high potential flooding has investigated after discharge routing of the sub-basins in the main channels to the outlet. The indicator watershed of Kasilian as the study area in the northern flank of the Alborz range divided to 3 sub-basin which was processed geometrically using GIS and Hec-HeoHms extension. With using HEC-HMS model and emission of individual repetition of the sub-basins, the homogenous flood hydrographs have gained in relation to the recorded precipitation calculated for different sub-basins. With consequent emission of sub-basins during the processes of the model running, total discharges of the basin calculated after flood routing in the main channels with ignoring of the sub-basins. Finally, sub-basin with most effective in flood production has determined as a high flooding basin. The flood routing in channels have shown that the rate of flood occurrence has no related the discharges of the channels.