An algorithm of mixed H2/H∞ based LMI algorithm for damping inter-area oscillations is investigated. A feedback controller is designed for such problem, which takes the uncertainty of power system parameter into account. In order to gain desired dynamic and steady state qualities, a pole-placement technique based on Linear Matrix Inequalities is used. At last, simulation of 4-machine 2-area system using this method is carried out, which proves the validity and robust of the designed controller. Different operating conditions are considered to validate the adaptability of this algorithm.