Impaired fasting glucose (IFG) is one of significant risk factors of developing diabetes. The persons with IFG are, thus, an important target group for primary prevention of diabetes. It is well known that plasma homocysteine concentration may be increased in poor folate nutritional status. Elevated level of plasma homocysteine is considered as a marker of enhanced oxidative stress. In addition, the protective effect against oxidative stress may be diminished in poor antioxidative nutrient status as vitamin C. It is, therefore, important to maintain adequate nutritional status of folate and vitamin C in the patients with type 2 diabetes or IFG. This study was performed to determine the effects of supplementation of folic acid or vitamin C on plasma concentrations of homocysteine, oxidized LDL, and lipids and on the activity of plasma anti-oxidative enzyme in patients with IFG. A total of 97 patients with IFG were participated voluntarily with written consents. They were divided into one of the four experimental groups; Control (C), Folatesupplemented (F), Ascorbate-supplemented (A), and Folate plus ascorbate-supplemented (FA). The subjects in C were taken placebo, those in F were supplemented 1 mg of folate, those in A were received 1,000 mg of vitamin C, and those in FA were given 1 mg of folate plus 1,000 mg of vitamin C daily for 4 weeks. No change in plasma concentrations of vitamin C, lipids, and oxidized LDL and the activity of GSH-Px were observed in vitamin C-supplemented group (A + FA) and folate-supplemented group (F + FA) compared to the placebo group (C + A). Only the folate-supplemented group (F + FA) had significantly increased average serum folate concentration and lowered plasma homocysteine concentration compared to the placebo group (C + A). Thus, it should be recommended the patients with IFG to increase folate intake through diets and, if it is not sufficient, to take folic acid supplements to prevent the development of complications induced by hyperhomocysteinemia as well as oxidative stress.