It is an important problem in multi area power systems to attenuate the frequency deviation due to la change. Since power systems are spatially dispersive, decentralized control is more practical than lump control. In this paper, we propose two robust decentralized control schemes for this problem. The difficul with this problem is that the linearized model of a multi area power system with frequency deviations outputs is an MIMO system that has a transmission zero at s=0. The key idea is to change the output one subsystem so that the resulting new plant does not have any zero at s=0. The new outputs are chos, carefully so that the regulation of the new outputs can guarantee the regulation of all frequency deviatio and tie-line power. Further, parameter uncertainty is taken into account in the controller design to achie robustness. Simulation shows the proposed methods are effective.