Abstarct: Flood is one of the natural disasters that its frequent occurrence in Iran has caused a lot of financial losses. Identifying areas with a risk of flooding and being aware of the probability of occurrence or return periods of flood events can be of great helpful in reducing the damage caused by this natural phenomenon. In order to predict the complex hydraulic phenomena, it is necessary to use simulation and modeling techniques. In this study, with the help of HEC-RAS, GIS and HEC-GEO RAS, the flood plain area, depth and average velocity on the rivers of Atrak Oliya (Sheikh Amirnlou River, Qoljogh River), Chaylag River, Gleyan River and part of the Atrak River with Different return periods was simulated. Simulation results on flood level showed that the flood surface area has a direct relationship with the flood return period. It also showed that very slight variations in the Manning roughness coefficient could lead to a difference of at least 1000 square meters of land at the level of flood landing. The depth, velocity, power and shear stress of the current are also increased by increasing the length of the return period. In order to estimate the damage, the average depth-damage function of Asia, as published by the European Research Center, was used. Then, in the GIS environment, the damming functions and flood simulation maps were combined and the damage raster map was calculated for residential areas with return periods of 10, 25, 50, 100, 200, and 500 years old. Using the damage map, you can get the amount of damage caused by the flood at any point. The results showed that for floods with 10, 25, 50, 100, 200 and 500 years returns, there are 41, 174, 275, 498, 643 and 760 return damages respectively. The average damages for these buildings and their content are 51, 53, 54, 52, 55, 56 percent.