BACKGROUND: Green-tea polyphenol (GTP) is a well known antioxidant with favorable effect on blood glucose and lipid level in animal models. We were to investigate the effects of GTP on plasma glucose, lipid and antioxidant systems in patients with type 2 diabetes. METHODS: We recruited non-complicated type 2 diabetic patients with stable glycemic control by oral hypoglycemic agents. Subjects were randomly assigned to GTP group or placebo group for 12 weeks. Fasting plasma glucose (FPG), HbA1c, C-peptide, lipid levels, liver function test, renal function test, urine microalbumin, malondialdehyde (MDA), superoxide dismutase (SOD) and glutathione peroxidase (GSHPx) activities were measured at baseline and after 12 weeks of treatment. RESULTS: At baseline, there were no significant differences in age, body mass index, duration of diabetes, dietary status, HbA1c, total cholesterol, LDL cholesterol, and HDL cholesterol levels between GTP and placebo group. However, FPG levels and triglyceride levels were significantly different between GTP and placebo group at baseline. In both of GTP and placebo group, there were no significant change after 12 weeks of treatment in FPG, HbA1c, total cholesterol, LDL cholesterol, triglyceride, levels of MDA, and GSHPx activities. SOD activities significantly increased after 12 weeks of treatment in both of GTP and placebo group. The increase of SOD activities were significantly higher in GTP group than in placebo group (P = 0.01). CONCLUSIONS: Supplementation of green tea polyphenol increased antioxidant activity in type 2 diabetic patients. The effect on plasma glucose and lipid level was not significant but should be confirmed in further large scaled studies.