Semi-active control systems and semi-active fluid viscous dampers as a subset of these systems, have shown their ability to reduce seismic responses of building, one of the most important advantages of these dampers is ability of adjusting with structures behavior to overcome the earthquake forces. In this thesis, multi-objective optimization of operation of semi-active fluid viscous dampers in reduction of seismic responses of buildings subjected to horizontal components of earthquake accelerations is studied. The optimum number and position of dampers are considered as decision variables and structural responses such as maximum story acceleration, maximum last story displacement and maximum story drift are considered as objective functions, should be minimized. The optimal solutions show that although viscous dampers reduce the structural responses, however, a certain configuration of dampers in specific locations of the structure could be efficient for reduction of the structural responses.