The protective effects of seleno-L-methionine (SeMet) on oxidative stress in pancreatic islets were investigated with a short-term nicotinamide (NA) and streptozotocin (STZ)-induced diabetic mouse model. ICR mice were intraperitoneally injected twice with 100 mg/kg STZ and 120 mg/kg NA at a 1-d interval and were then orally administered 158 µg Se/kg SeMet with free access to a selenium-deficient diet for 5 weeks. Administration of SeMet significantly improved the levels of glycated hemoglobin (HbA1c), non-fasting and oral glucose tolerance-tested (OGTT) blood glucose, plasma adiponectin and hepatic glycogen that deteriorated by NA/STZ treatment. However, supplementary SeMet did not restore non-fasting plasma insulin levels in NA/STZ treatment group and significantly suppressed OGTT plasma insulin levels in the control group. Although SeMet significantly suppressed 8-hydroxy-2′-deoxyguanosine density in pancreatic islets, SeMet did not restore insulin density. The hepatic and pancreatic mRNA levels of glutathione peroxidase 1 (GPX1) increased by NA/STZ treatment or SeMet administration. These results suggest that although a physiological level of SeMet improves glucose tolerance by exhibiting insulin-mimetic activity in a short-term induced diabetic mouse model under insufficient Se status, the suppression of pancreatic oxidative stress with the induction GPX1 by SeMet supplementation is unlikely to restore insulin storage and secretion.