Recently, semi-active control using megnetorheological (MR) dampers that combines the advantages of both active control and passive control has received considerable attention. However, how to design the corresponding control system is still a challenge task. This study proposes a genetic algorithms-based strategy for designing the fuzzy controller for the multi-input-multi-output (MIMO) structure-MR dampers system. This strategy determines the command voltages of MR dampers from the acceleration responses of the structure. Genetic algorithms are employed to adaptively design and optimize the fuzzy rule bases. The experimental studies have demonstrated the effectiveness and efficiency of the fuzzy control systems based on the proposed design strategy in reducing the responses of a vibrating structure.