Angelan isolated from Angelica gigas Nakai inhibits tumor growth and metastasis by enhancing immune functions of macrophages, dendritic cells, and B cells. Here, we report that angelan can inhibit autoimmunity in non-obese diabetic (NOD) mice. Although 80% of the NOD mice had developed diabetes by 24 weeks of age, none of the angelan-treated NOD mice developed diabetes. The mean glucose levels were 118 mg/dl in angelan-treated mice and 506 mg/dl in control NOD mice. Histological examination of the pancreatic islets revealed that most of the islets isolated from angelan-treated mice were less infiltrated with lymphocytes compared with those of control mice. Spleen cells from diabetic NOD mice could adaptively transfer diabetes into NOD.scid mice, but those from angelan-treated NOD mice did not, suggesting that angelan caused the spleen cells to lose the ability to destroy $eta$ cells. However, angelan did not affect cytokine production of spleen cells. These results suggest that angelan has dual immunomodulatory functions, i.e., immunostimulation in tumor-bearing mice and immunosuppression in autoimmune diabetic mice.