Designing the important structures with high limit targets is not easily accessible while the customary seismic-resistant systems are in use. Hence, control systems are employed to reach structural performance goals. This thesis concentrates on optimum seismic design of structures that are equipped with magnetorheological (MR) dampers in order to decrease structural response and the structure’s life cycle costs. MR dampers are categorized into semi-active control systems in which forces are dependent on the input voltage. In this study the modified behavioral model of Bouc-Wen is used to simulate the behavior of MR dampers. This model has higher accuracy rate for simulating the behavior of MR dampers in comparison to similar models. Fuzzy control is used to control the input voltage of the damper in a way that the voltage will be chosen and applied in accordance to structural responses. Newmark-Beta method is utilized to calculate the MR damper equipped structure’s response considering the fact that it has a nonlinear behavior. Interstory drift limitations of ASCE 07-16 were placed as the allowable performance limits of the structure. Numerical results of a steel structure case study of 11 story showed magnificent effect of MR dampers in decreasing structural responses up to 26 and life cycle cost up to 11 percent. Also, considering various layouts of dampers shows that increasing the cost of dampers does not necessarily lead to lessening of life cycle costs which portrays the necessity of proposing an optimized plan for using MR dampers in the structure. The results of optimized seismic design showed that, aiming for a decrease in damper costs, drift limitations are observed with 27 percent cost decrease in comparison to unified distribution of dampers and when the goal is to minimize life cycle costs, by the 26 percent decrease rate of the damper costs, the total life cycle cost decreased 17 percent in comparison to the unified distribution of dampers. Analysis results demonstrated that the proposed fuzzy control has superior performance when it is intended to minimize the total expected life cycle costs. Keywords MR damper, Fuzzy control, Seismic optimization, Genetic algorithm, nonlinear dynamic analysis, Analysis Life Cycle Cost