Abstract: This study aimed to observe the effects of betulinic acid (BA) on the lymphocyte and macrophage in mice. BA was synthesized by oxidizing betulin from the bark of the white birth with Jone′s reagent, followed by selective oxidation of the resulting betulonic acid with NaBH₄. 168 KM mice (6 weeks old) were orally administered the BA for 14 days with the dose of 0, 0.25, 0.5 and 1 mg·kg^-1 body weight. The effect of BA on the lymphocytes proliferation, thymocyte and splenocyte subsets, the number of plaque forming cells (PFC) and the phagocytic activity of the macrophages were evaluated. Oral administration of BA was found to significantly increase the thymus and spleen indices, lymphocytes proliferation induced by Concanavalin A or lipopolysaccharide, the percentage of CD4⁺ cells in thymus as well as the percentage of CD19⁺ B cells and the ratios of CD4+/CD8+ cells in spleens The number of PFC and the macrophage phagocytic activity was also enhanced. The results suggest that BA enhances mouse cellular immunity, humoral immunity, and activity of macrophages.