Drug interactions with food, on occasion, lead to serious nutritional and functional changes in the body as well as alterations of pharmacological effect. It, therefore, should be necessary to take drug interactions with food into consideration for effective and safe therapeutics. Diabetes mellitus is a heterogeneous group of disorders characterzed by abnormal glucose homeostasis, resulting in hyperglycemia, and is associated with increased risk of microvascular, macrovascular, and neuropathic complications. However, the precise mechanism of diabetes mellitus remains unclear. Three basic objectives in the care of diabetic patients are maintaining optimal nutrition, avoiding hypo- or hyperglycemia and preventing complications. Laminaria japonica is a brown macroalgae which can be used as a functional diet due to high content of diatery fiber. The purpose of this study was to investigate the effect of Laminaria japonica diet on the pharmacokinetics of metformin which are frequently used in the treatment of diabetes. Diabetic rats induced by streptozotocin were employed in this study. Blood concentrations of oral hypoglycemic agent, metformin, were measured by HPLC and resultant pharmacokinetic parameters were calculated by RSTRIP. The mechanisms of drug interaction with food were evaluated on the basis of pharmacokinetic parameters such as $k_{a},;t_{1/2},;C_{max},;t_{max}$, and AUC. Administration of metformin in normal and diabetic rats treated with Laminaria japonica diet showed significant decrease in AUC, $C_{max},;and;k_a$, and increase in $t_{max}$, compared to those with normal diet. This might result from adsortion of metformin on components of Laminaria japonica, causing delayed absorption. The oral glucose test showed that Laminaria japonica diet could lower blood glucose level probably through either inhibiting the activity of disaccharidases, intestinal digestive enzymes, or delaying the absorption of glucose. More studies should be followed to fully understand pharmacokinetic changes of metformin caused by long-term Laminaria japonica diet.