In some applications，the DC-side voltage may do not match with AC-side voltage of modular multilevel converter （MMC）. To solve this problem，an optimized design method is proposed to determine the number of cascade sub-modules in an arm of MMC. With the proposed design method，MMC can be integrated into the AC grid of lower voltage even when the DC- side voltage does not match the AC-side voltage，so that the number of sub-modules in MMC can be reduced and without using transformer. Therefore，the size and the cost of MMC can be significantly reduced. An simulation module is built in PSCAD/ EMTDC to verify the proposed method.