In this paper control of three phase UPS inverter with cascaded PI controller and load current estimation via disturbance observer are presented. Pure sinusoidal waveform with specified amplitude and frequency is required for UPS inverter under variety of loads. Load current acts as disturbance on the plant. To increase the disturbance rejection of the plant, the load current is adopted as feed forward loop to the voltage controller. Disturbance observer method is relatively simple method in comparison with Luenberger observer. The proposed algorithms are successfully presented with simulations and verified by experimentally.