A control scheme is proposed to optimize the Modular Multilevel Converters (MMC) efficiency based on harmonic circulating current injection. With the presented efficiency optimization scheme, the MMC loss is reduced by about 3% to 4% compared to the conventional circulating current eliminating method under the rated 1000MW. The relationships between the losses and the harmonic circulating currents in MMC are also derived. Verification results show correctness of this control scheme.