An increase in oxidative stress may contribute to the development of oxidative protein damage in streptozotocin diabetic rats. In the present study, the influence of alpha-lipoic acid supplementation on plasma protein carbonyl, plasma thiol, and plasma lipid hydroperoxide levels was examined in order to characterize the relationship between the oxidative stress and the oxidative protein damage. Rats were randomly divided into three groups of equal body weight. Chronic hyperglycemia was induced by intravenous streptozotocin injection in both the group of male Wistar rats to be supplemented with alpha-lipoic acid and the group that was not to receive alpha-lipoic acid. Nondiabetic rats formed the control group and received a saline injection. In streptozotocin diabetic rats with and without alpha-lipoic acid supplementation, plasma carbonyl levels were significantly increased, while plasma thiol levels were significantly decreased compared with those of the control group. Plasma lipid hydroperoxide levels were significantly increased in diabetic rats without alpha-lipoic acid supplementation compared with those of the controls, but the lipid hydroperoxide levels in the alpha-lipoic acid supplemented group were no different from those of the controls. In streptozotocin-diabetic rats, oxidative stress was significantly decreased in the alpha-lipoic acid-supplemented group. The results of this study suggest that alpha-lipoic acid, by decreasing oxidative stress, may be effective in preventing oxidative protein damage, which may contribute to the development of diabetic complications.