This paper proposes a transformer-less modular multilevel DC/DC converter. The proposed converter can be used to interconnect two HVDC systems with different dc voltage levels and realize bidirectional power flow. Based on the arm energy balance condition, the optimal design arming at minimum inner circulating current is presented. A power close-loop control method is adopted to regulate the converter transmission power. In the method, since the ac component of the reference voltage for each arm is high frequency, the volume of sub-module (SM) capacitor is reduced. Finally, a 320 kV/160 kV 80 MW modular multilevel DC/DC converter is built using PSCAD/EMTDC software. The results validate the operation principle of the proposed converter.