Fructus Arctii (great burdock achene) is the dried ripe fruit of Arctium lappa L. (family Asteraceae) and is included in the Chinese pharmacopoeia. It has been reported that the clinical use of Fructus Arctii resulted in a satisfactory hypoglycemic effect in diabetic patients. This study aimed to investigate antidiabetic activity and mechanism of total lignans from Fructus Arctii (TLFA) in KKAy mice, a spontaneous Type 2 diabetic rodent model that exhibits marked obesity. In this study, KKAy mice were gavaged once daily with solvents (0.5% sodium carboxymethyl cellulose), TLFA (250 and 125 mg/kg), or metformin (200 mg/kg) for 11 weeks, and C57BL/6J mice treated with saline solution (0.9%, w/v) were used as normal control. The results indicate that TLFA has dual effects of hypoglycemia and weight loss, and administration of TLFA in KKAy mice could decrease fasting blood glucose, glycated hemoglobin, and body weight; improve oral glucose tolerance; increase high density lipoprotein cholesterol; and decrease triglycerides and free fatty acid in mice serum. Its efficacy may associate with multiple mechanisms of action such as stimulation of insulin secretion, activation of phosphatidylinositol 3-kinase/protein kinase B, and adenosine-monophosphate-activated protein kinase signaling pathway, decreasing leptin.