[Objective] We studied the immunomodulatory potential of Lactobacillus casei Zhang on preventive and therapeutic effects.[Methods] A murine model of peanut allergy was established and fed with L. casei Zhang. Then we measured the levels of IgE, IgG1 and IgG2a antibodies in blood serum, histamine and IgA antibody in feces, and cytokines and CD4⁺Foxp3⁺ Tregs cells.[Results] Intragastric administration with L. casei Zhang, prior or after allergic sensitization, increased levels of serum allergen-specific IgG2a and fecal total IgA antibodies. Additionally, systemic anaphylactic symptoms and Th2 responses such as total and specific IgE antibodies, histamine release and IL-4 production were decreased. In contrary, L. casei Zhang had no obvious influence on the serum IgG1 antibody and other cytokines (IL-10, IL-12 and IFN-γ). Successful immunomodulation by pre-and post-treatment were further demonstrated by the increase of IFN-γ/IL-4 ratio in vitro and enrichment of CD4⁺Foxp3⁺ Tregs in spleen and mesenteric lymph nodes.[Conclusion] L. casei Zhang could shift the Th2 dominant response to a Th1 response, which finally play a role in preventing and treating peanut allergy.