We performed a randomized double-blind placebo-controlled trial to determine whether Laminaria japonica extract (LJE) supplement modulates blood glucose, serum lipids and antioxidant systems in type II diabetic patients. We also measured critical parameters assessing safety in liver and kidney functions after LJE supplement. A total of 37 patients (18 males and 19 females) were randomized to either LJE group or placebo group. The treatment group received four 350 mg of LJE capsules (1.4 g, total) per day for 12 weeks. The placebo group received the same dose of cellulose capsules. Baseline characteristics regarding general life style and dietary intake pattern were similar between the two groups. There were no significant influences of LJE supplement except for waist circumference on anthropometric parameters. As the whole, 12 weeks of LJE supplement resulted in a little decrease in fasting blood glucose (FBG) and glycated hemoglobin (HbA1c), but a significant decrease was not observed. Total cholesterol, LDL-cholesterol and triglyceride concentrations were significantly (p<0.05) lowered in LJE group. The antioxidant enzymes, glutathion peroxidase (GSH-px) and superoxide dismutase (SOD) levels were elevated in the LJE group (p<0.05) compared to the placebo. The increase of these enzymes was associated significantly with the decrease of MDA concentration (p<0.05). Furthermore, LJE supplement showed no adverse effects on the functions of liver and kidney. Findings from this study suggest that LJE supplement can help improve serum lipid status in type II diabetic subjects without adverse effects.