Distributed Energy Resource (DER) is a small-scale power generator which can be distributed in near power consumption areas. A high ratio of DERs increases management complexity and also decreases supply stability of electric power. However, if DERs are suitably used, they can improve reliability shown in conventional electric power generation. Moreover, they can also eliminate the size and number of power lines that must be constructed. Since the existing electric power system of Korea is highly centralized, it needs the decentralization strategies by allowing DERs to participate in the existing power trading market. In this context, Korea government is on the stage of deciding national electricity laws about trading DERs. Of course, various operational strategies should be defined together. In this paper, we suggest the brokerage trading system of the aggregated DERs, which makes DERs to join in the existing electric power system. Since May 2016, we are in the progress of research and develop the aggregator system for trading DERs in form of the wholesale.