Objective: To explore the effect of Tripterygium polyglycoside (TP) associated with nicotinamide on the islet cell function, immune parameters and lipoperoxide (LPO) in adult patients with late-onset autoimmune diabetes mellitus (LADA) Methods: Thirty-six cases of LADA were randomly divided into three groups: TP group (n= 12), treated with TP plus orally taken metformin; combined treatment group (n =12), treated with TP combined with nicotinamide and metformin, and control group (n = 12) treated with metformin alone. They were followed-up for 18 months. Results: (1) Compared with the control group after 9months of treatment, postprandial plasma glucose and LPO in combined treatment group were decreased (P ＜0.05), and the postprandial C-peptide was higher (P＜0.05). At the 18th month, the value of postprandial C-peptide in the TP and combined treatment group was higher than that in the control group. The slL-2R level of both TP and combined treatment groups were lowered (P＜0.01); (2) Islet cell antibody (ICA) positive of 5 cases in the TP group and 6 cases in the combined treatment group got converted to the negative respectively, while only one in the control group at the time (P＜0.05) ; (3) The level of LPO in the combined treatment group was significantly lower than that in the TP group at the 18th month of treatment (P＜0.05).Conclusion: TP combined with nicotinamide played a role in immunity regulation, decreasing the titer of islet cell antibody and slL-2R, which also reduced the production of LPO and had a tendency to improve islet cell function in early LADA patients.