The determination of the optimal locations of controllers in active or semi active structural control has received very little attention. The optimal placement, compared with the non optimal case, provides control performance with a fewer number of controllers and smaller control force. In this research, the effect of the locations of the controllers on the control force and control performance was studied. This project uses the ‘Pole-Assignment’ method for the purpose of determining control forces, and for considering the semi active state. In the semi active control state system, the control force is determined by specifying a value to produce this state. During this study, two optimization methods have been used for active control. In the first method, all cases are studied to find the optimal case. Then several evaluation criteria have been used to find an optimal case. The second method is based on defining a performance index related to each story. In this method, with a series of repeatable operations, stories with a maximum index have been selected as optimum. The numerical examples show that the results of the first and second method are somewhat different. In addition, the findings of this study indicate that the number of controllers can be reduced in most cases. General results have been obtained for the optimal placement of controllers in common structures.