Energy efficiency of DC distribution systems is researched in this paper. Efficiency calculation models of feeders and loads are established, efficiencies of AC/DC, DC/DC and DC/AC are analyzed. Moreover, energy efficiencies of an AC system and two DC systems, monopole and bipolar, are calculated and compared. The efficiency improvement of office building supplied by DC power system compared to supply by AC power system is demonstrated. From analysis, it is showed that the energy efficiency is higher in DC distribution system than AC distribution system.