Miglitol, a 1-deoxynojirimycin derivative, is an α-glucosidase inhibitor. In the present study, the effects of acute (single-dose) and chronic (8-week) oral administration of miglitol in Goto-Kakizaki (GK) rats, an animal model of type 2 diabetes, were investigated. Dose-dependent decreases in incremental blood glucose concentrations integrated over a period of 2 h (ΔAUC0–2 h) for values of blood glucose after sucrose-loading in miglitol-treated GK rats were observed following an acute oral administration of miglitol (1, 3 or 10 mg/kg body weight). At 10 mg/kg, the ΔAUC0–2 h of blood glucose was decreased by 45 % compared with the control group. Following the oral administration of miglitol in a dietary mixture (10 mg, 20 mg or 40 mg miglitol/100 g control diet) for 8 weeks, the ratio of HbA1c at 8 weeks compared with 0 weeks in GK rats treated with 40 mg miglitol/100 g control diet miglitol was significantly decreased compared with control GK rats without changes in body weight. In oral glucose tolerance testing, miglitol caused a slight decrease in the ΔAUC0–2 h of plasma glucose concentration. In addition, miglitol treatment slightly inhibited the reduction in β-cell mass, and lessened the irregular contours and fibrosis of the islets in GK rats. These results indicate that miglitol ameliorates the hyperglycaemic state of GK rats and the impaired function of the pancreatic islets, as well as preventing the degeneration of islets in GK rats.