We investigated the effect of vitamin E isoforms on the glucose tolerance in high-fat-induced obese mice. Eighteen male C57BL/6J mice aged six weeks were divided into three groups and fed a high-fat diet without vitamin E (HF), HF supplemented with 0.1% α-tocopherol (α-Toc) or HF supplemented with 0.1% γ-tocotrienol (γ-T3) for ten weeks. Oral glucose tolerance and insulin tolerance were tested after nine weeks of experimental diet feeding. Although growth rate and food intake were not significantly different among the three groups, addition of γ-T3, but not α-Toc, improved glucose tolerance and insulin sensitivity induced by the high-fat diet in obese mice. Fasting insulin levels in the γ-T3 group were lower than those in other high-fat groups, and the level of insulin secretion following glucose loading was significantly higher in the γ-T3 group than in the HF group. Histological findings showed that islets of the γ-T3 group were sufficiently active to secrete insulin in spite of high-fat feeding. These results suggested that γ-T3, but not α-Toc, protected islets against lipotoxicity and could be effective in treating type 2 diabetes.