The transformer-less modular multilevel converter (MMC) DC-DC converter can achieve direct DC-DC conversion without an intermediate AC transformer. In this paper, a transformer-less MMC DC-DC converter topology with bi-directional DC fault blocking capability is proposed. The proposed converter comprises half bridge submodules (HBSMs) and full bridge submodules (FBSMs). Use of FBSM eliminates the large AC filter which is normally required in the transformer-less topologies. Moreover, the proposed topology has the capability to block the DC fault at either DC side. Once an DC fault is detected, the switches in SM are blocked and the capacitors of HBSM in one DC side are inserted in the fault circuit path. The fault current charges the inserted capacitors and thus reduces to zero quickly. The performance of the proposed topology is validated in a study system using a real-time simulation tool.