In this study, an instantaneous optimal control performance index for structures with magneto-rheological (MR) dampers is analytically defined. Absolute acceleration and absolute velocity feedback are implemented to this simple performance index. The performance index is another application of instantaneous optimal control algorithms. A three story structure incorporating an MR damper is presented as a numerical example. Modified Bouc Wen damper force model is used for obtaining the MR damper force. The electrical current need for the operation of the MR damper is taken into account by using a piece wise invertible MR damper model. Near fault earthquakes are used in the dynamic simulations. Seismic energy-based evaluations are carried out. Uncontrolled structural response reduction performance and power requirement of the MR damper is investigated. The analysis shows that proposed performance index is effective in reducing uncontrolled structural seismic responses.