: Spillways are used in dam projects to convey flood flows and also to prevent destructive damages of downstream hydraulic systems. Siphon spillway is a kind of dam spillways, employed in a number of dam reservoirs. This spillway is applied while the space for constructing other types of spillway is restricted or when there is a need to pass great values of flood discharge with a limited head. One of the most important defects of this spillway is a complicated fluid-structure interaction (FSI), due to the establishment of different flow regimes inside the spillway. This situation becomes more complicated when the spillway is not connected to the dam, working individually. Despite the importance of the operation of these hydraulic structures, there is a lack of comprehensive research, investigated the hydraulic behavior of these structures and the flow field around and inside them. In the present study, different flow regimes (sub-atmospheric flow, two-phase flow and black-water flow) and their effects on the hydraulic characteristics of siphon spillways were investigated. For this purpose ANSYS software was applied to simulate the fluid-structure interaction in such spillways. To validate the obtained flow regimes, using numerical modeling, experimental results of former researches were used. According to the verification of numerical models, a flood hydrograph was assumed as the inlet boundary to simulate the transient situations. Then, the forces of the flow were transmitted to the structural solver in specified time steps. To transmit the flow forces to the structural solver, the structural material, structural supports and the thickness of siphon structure were specified. Accordingly, the outputs including bending stresses and deflections were compared with the allowable values given in the steel structures standards. Results indicated that the pressure fluctuations are mostly greater beneath the inlet and throat of the spillway compared to the other sections. Furthermore, the maximum stresses and deflections are obtained in the downstream limb of the spillway. Based on a structural analysis and by applying some changes in structural supports and body of the structures, the structural design of the siphon spillways were optimized. Kewwords: Siphon spillway, Fluid-structure interaction, Computational fluid dynamics, Black-water flow