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The antioxidative activities of low-caffeine tea extract (LCTE) prepared from green tea leaves were compared with those of green tea extract (GTE). The yields of GTE and LCTE from tea leaves were 17.3% (w/w) and 10.2% (w/w), respectively. The catechin and caffeine contents of LCTE were ca. 43% and ca. 8% those of GTE, respectively. β-Carotene and α-tocopherol were detected only in LCTE at levels of ca. 0.3% (w/w). LCTE showed the same levels of iron-chelating and radical scavenging activities as GTE in vitro. To study antioxidative activities of LCTE in vivo, male ddY mice were given the following different diets for 6 weeks: normal diet, or that supplemented with 1.0% GTE, with 0.5% LCTE, or with 1.0% LCTE. Diets supplemented with 1.0% GTE, 0.5% LCTE, and 1.0% LCTE tended to lower lipid peroxide contents in mouse plasma in comparison to mice fed the normal diet. In the mouse liver, lipid peroxide contents in 1.0% GTE, 0.5% LCTE, and 1.0% LCTE groups were significantly decreased compared to those of the normal group. Retinol and α-tocopherol contents in the plasma of mice fed diets supplemented with 0.5% LCTE and 1.0% LCTE were significantly higher than those of mice fed the normal diet. These results suggest that supplementation of the diet with LCTE could elevate resistance to oxidative stress in vivo.