Background and Purpose: In the development of vaccines, it is important to increase the antigen of the vaccine. We have confirmed the potential of Morus alba L., a Toll-like receptor 4 (TLR4) activator, as an adjuvant as a method for increasing the antigenicity of the vaccine. Experimental Approach: TLR4 is highly expressed on immune cells, therefore, the present study was undertaken to investigate the underlying immunological processes of Morus alba L. as an oral adjuvant in a mouse model. Mice were immunized with two types of antigen (ovalbumin (OVA) or inactivated influenza vaccine) combined with Morus alba L. Key Results: In the OVA antigen model, the co-administration of Morus alba L. significantly promoted phagocytosis in murine peritoneal macrophages. The maturation and function of dendritic cells were significantly up-regulated the expression of MHC-II, CD86, and CD80 compared to OVA alone. The titers of OVA-specific IgG, IgG subtype responses, and IgA were increased, the proliferation of T and B-cells was markedly enhanced, and the production of IL-12, IFN-, and IL-4 was better than OVA alone. In the inactivated influenza H1N1(PR8) vaccine model, the co-administration of Morus alba L. with vaccine significantly increased the IgM, IgG, and IgG subtype responses. The immunization of mice with Morus alba L. adjuvant reduced mortality in mice after a lethal challenge with influenza virus. Conclusion and Implications: Morus alba L. is a novel mucosal adjuvant for vaccination that provided safe and effective adjuvant effects and successfully induced both serum and mucosal antibody responses and cell-mediated responses.