Modular multilevel topology based DC-DC converters(MMDC) have attracted much attention in the industrial and academic areas. However, compared with the traditional MMC in high voltage dc transmission(HVDC), due to the new operating principles and modulation methods, the electrical and thermal stress distribution of MMDC is not clarified yet, which could threaten the reliability and lifetime of MMDC. Thus, it is essential to investigate the thermal performance of power devices in MMDC to provide guidance on thermal design and active thermal control. In this paper, the power losses of submodule devices are calculated and the thermal model is built first. Then, the concept of dc current modulation ratio is defined, based on which the relationship between power losses and thermal distribution is revealed. As the dc current modulation ratio decreases, the thermal distribution imbalance gets more serious. Finally, the analysis is verified by an MV MMDC simulation model and laboratory prototype.