Nowadays, modular multilevel converters (MMC) are widely used in offshore wind power DC transmission systems. However, the development of offshore wind power is strongly constrained by large volume, heavy weight and high cost of MMC. Topology of diode rectifier (DR) in series with MMC is proposed due to its lower cost and higher reliability. In this paper, black start-up strategy of MMC in series with DR DC transmission system is presented and introduced in detail. There are four steps in the start-up process. First of all, onshore hybrid MMC is charged and triggered. Secondly, offshore MMC is charged and triggered. Offshore MMC establishes AC voltage and wind turbines (WTs) start up. After that, offshore MMC reduces AC voltage. Wind turbines operate in low-voltage-ride-through (LVRT) mode and onshore MMC rises up DC voltage. Finally, offshore MMC raises AC voltage and wind turbines return to normal operation, meaning that black start-up process is finished. Simulation results confirm the proposed start-up strategy.