Free radicals are considered to be important causative factors in the development of cancer and cardiovascular diseases. This relationship has led to interest in evaluating the antioxidant capacities of many dietary supplements. Fermented soybean extract is produced by symbiotic fermentation of organic soybean with 20 types of Lactobacillus and yeast. In vitro and in vivo models are used in this study to evaluate the antioxidant effect of fermented soybean extract. Several in vitro models are used to detect the antioxidant capacity of the fermented soybean extract, which is compared to vitamin C and Trolox. The results demonstrate that the fermented soybean extract has strong antioxidant activity against unsaturated fatty acid peroxidation compared to vitamin C and Trolox. By the means of the test system developed by Y. Toshiki et al., it is shown that the fermented soybean extract can function both as an antioxidant and as a free radical acceptor that can convert free radicals into harmless substances through an energy-decreasing procedure. An in vivo study examines the effects of fermented soybean extract on the activity of antioxidant enzymes. The activities of the antioxidant enzymes (AOE) including total superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX) are measured in liver, kidney, and brain from male Sprague-Dawley rats. The activities of CAT, SOD, and GPX are increased in the liver. However, the SOD activity is decreased in the kidney. SOD and GPX activities are decreased in the brain. These results lead to the conclusion that fermented soybean extract not only has antioxidant activity but also has an effect on the activity of antioxidant enzymes in liver.