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Recently, information devices such as computer systems play an important role in the society. These devices causes a malfunction and an outage by an instantaneous blackout, and restoring these devices takes a long time. Therefore, in order to supply the stable power, adoption of UPS (uninterruptible power supply) system is indispensable. And in order to enhance the reliability of the power supply, it is necessary to construct the redundancy system which combined with multi UPS system. In this paper, an autonomous decentralized control for parallel-connected single phase UPS system to enhance the reliability of the overall power system without any interaction for each UPS system is proposed. A quasi dq transformation method is used to detect the instantaneous pq value for the single phase voltage and current waveforms. Simulations were carried out in the case that the some UPSs were connected to the same load line, and the load variation or the disconnection of the UPS was occurred. The autonomous load balancing characteristics was confirmed for various conditions.