"DC Micro-grid" is a novel power system using dc distribution in order to provide super high quality electric power. The power is distributed through dc distribution line and converted to required ac or dc voltages by converters placed near loads. The load side converters do not need transformers by choosing proper distributed dc voltage (±170V). The dc distribution line is composed of 3 power lines: +170V line, neutral line and -170V line. The voltages of ±170V have to be balanced to supply high quality power. In this paper, a voltage balancing control method for dc distribution is proposed and studied. Computer simulation results demonstrated that dc micro-grid was able to supply both ac and dc power to loads simultaneously and stably by 3-wire dc distribution and load side converters.