Today, the management of flooding and surface runoff in urban areas is very important. In this study, adequacy of surface water collecting network in Masal city for passing the design flood is considered. The basic information about the climate, topography and land use provide for simulation of surface water collecting network using MIKE SWMM computer model. Hydrologic simulation in RUNOFF block was performed by entering the physiographic characteristics, pattern and the time distribution of precipitation for 10 return periods. For hydraulic simulation, the inputs from junctions and conduits characteristics were entered to EXTRAN block, and also the geometrical properties of the channel, grids, boundary conditions and outlets were fed to the model for simulations. The results showed that existing condition has not been able to pass design flood and this matter causes surcharge and flooding conditions in location of junctions more than 360 minutes. Therefore with technical and application considerations and with a trial and error procedure, adequate sizes of channels for passing design flood was determined so that in occurring of design flood, surcharge and flooding conditions have been omitted completely or become minimum (less than 15 minute) and according to that, the typical sizes of channels for passing design flood were proposed. Also in this, the sensitivity analysis of model was examined in RUNOFF block. Area and the percent of imperviousness with sensitivity index equal 2.