Abstract: Normally MMC (modular multi-level converter) is adopted in the flexible HVDC transmission system. The power loss of converter is one of the most important indexes to evaluate the performance of flexible HVDC system. In this paper, an analytical expression for the on-state and switching loss of MMC is derived. As can be seen from the expression, the power loss of converter is not only related to the number of MMC sub modules, but also the capacity and voltage level of the flexible HVDC transmission system. In this paper, a detailed investigation of the influence of the capacity and voltage level of HVDC system on the power loss of converter is carried out, so as to identify the optimal voltage class that reduces the power loss to a minimum. Simulation results show that the proposed method can be used to find out the voltage level that helps lower the power loss.