Generally, the rainwater harvesting systems are used as a strategy for the management of runoff and water supply during rainfall. Before transforming the rainfall in the form of evaporation or flooding, it can be collected and used with help of storage methods. There are many restrictions for implementation of rainwater harvesting systems, such as, the lack of space and the cost of installing these systems, and these are caused the optimization to be inevitable. Imperialist competitive algorithm (ICA) is a new method of optimization that uses from a social phenomenon and has an appropriate performance. In this paper, 4. The multi-objective imperialist competitive algorithm (MOICA) has been studied on part of the Tehran city, Iran. The results showed the high effectiveness of the optimal volume of rainwater harvesting systems on the peak discharge and flood volume, so that for a flood with a return period of 50 years, the peak flow reduced up to about 53 percent and flood volume reduced up to about 49 percent.