MENK, an endogenous opioid peptide has been reported to have many immunological and antitumor activities. So far the detailed mechanisms of antitumor through regulating DCs by MENK have not been elucidated yet. The aim of this work was to investigate the antitumor mechanisms of MENK via regulating DC. The monitoring methods, such as ELISA, MTS assay, CFSE, Real-time PCR and Western blot were included in our research. We found bone marrow derived dendritic cells (BMDCs) in 36 female C57BL/6 mice treated with MENK enhanced expression of key surface molecules, increased production of critical cytokines reduced endocytosis of FITC-dextran, upregulated TLR4 through MyD88/NF-κB signaling pathway and mounted higher antitumor activity. These observations were further supported by an enhancement of nuclear translocation of the p65NF-κB subunit involved in this process. Surprisingly, mu-opioid receptors were the main participants of this kind of activation, not delta-opioid receptors nor kappa-opioid receptors, and these interactions could be partly blocked by Naltrexone (a kind of opioid antagonist). In vivo study the activated CD4+, CD8+T cells and decreased ability to induce differentiation of Foxp3+ regulatory T cells were detected post treatment of MENK. Thus, it is concluded that MENK could exert antitumor effect through precisely regulating opioid receptor mediated functions of DCs. In addition, MENK treated DCs may serve as a new immunotherapy approach against tumor.