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In this paper, an improved nonlinear control strategy of the inverter output voltage for the three-phase UPS systems is proposed. At first, a nonlinear modeling including the output LC filters is derived from the power balance condition between the inverter output and load sides. Then input-output feedback linearization is applied to the nonlinear model, which can be linearized. The controller of the linearized model can be designed easily by linear control theory. The tracking control law is obtained with the pole placement technique. It is shown that the proposed control scheme gives high dynamic responses at load variation as well as a zero steady-state error.