Oil-adjuvant-inactivated vaccine is one of the most cost-effective vaccines used to protect ducklings against RA infection; however, it does not provide complete protection in very young ducklings with immature immune systems. In the current study, LMS was used as an immunopotentiator to improve the immune system in ducklings. Serum immunoglobulin (Ig)G titers and the secretions of both Th1-type (IFN-γ and IL-2) and Th2-type (IL-4 and IL-10) cytokines were higher in ducklings that had been vaccinated with LMS. In addition, a significantly higher T-lymphocyte proliferation rate was obtained with the addition of LMS. Furthermore, all of the ducklings vaccinated with LMS were protected against RA on the 9th day post-vaccination, whereas only 69.2% of the ducklings were protected in the group that did not receive LMS. These results suggest that LMS might be a useful adjuvant to enhance the immune response of ducklings. The use of LMS may also alleviate local injection lesions, caused by the oil-emulsion vaccine, by reducing the dose of the vaccine.