The use of retarding reservoirs is one of the methods of urban flood management structures to reduce the volume or intensity of floods as well as repulsed of flooding flows. In this research, the effect of intercity retarding reservoirs on flood peak discharge and the dimensions of conduits was analyzed in one of the subcatchment of Sari (Gozarkhan) for this purpose, after determine the designing rainfall with 5 years return period based on SCSII's method and the concentration time of subcatchments was calculated by using the experimental methods of Kerby and Kirpich, the simulation of the network without reservoirs was performed by using SWMM software and the flood peak discharge and the dimensions of conduits were determined then, with flood retarding reservoirs, re-simulation and the effect of reservoirs on peak of output discharge and the dimensions of conduits were determined. In order to determine the volume of flood retarding reservoirs, first by considering several quantity for volume and drawing output discharge and  variations graphs in relation to the volume of the reservoir, for each reservoir, the volume with the lowest rate of variation of the discharge was considered as the optimal amount. The results show that the retarding reservoirs reduce the flood rate in output node 3266, 86% and in output node 4004, 89%, as well as the dimensions of the conduits (average cross-section of conduits) reduce 57% in study area.