This paper addresses an LMI (linear matrix inequalities) based robust control methodology for designing of power system stabilizers (PSS). The PSS design problem is considered as a multi-objective control problem and formulated via a mixed H2/H∞ control technique, then for a given power system it is easily carried out to synthesis the desired robust controller by solving standard LMI. A single-machine infinite-bus system example is given to illustrate the developed approach. The results of the proposed control strategy are compared with conventional PSS design. The robust PSS is shown to maintain the robust performance and minimize the effect of disturbance and specified uncertainties.