Abstract: This paper studies the topologies of modular multilevel converter (MMC) sub-module with DC side fault blocking capability. By analyzing the working characteristics of half bridge and full bridge MMC sub-modules in DC side fault condition, a new sub-module based on reverse blocking devices was proposed. The topology can change the fault current path without inserting additional power devices into the normal current path of the sub-module. Compared with the existing sub-module topologies with DC fault blocking capability, the proposed topology has lower conduction loss under normal conditions. In order to verify the effectiveness of the proposed topology, an MMC converter simulation platform was built in the Matlab/Simulink environment, and the corresponding prototype was built to comprehensively verify the DC side fault blocking capability of the proposed sub module topology.