The relationship between the content of anthocyanin and the radical-scavenging activity in the seed coat of 17 varieties of black soybean was investigated. The effects when boiling were investigated of added iron reduced by hydrogen and of Fe2O3 on the radical-scavenging activity and on the seed coat color of black soybean. Although the composition of anthocyanin in the seed coat varied considerably with the variety of black soybean, the main anthocyanin was cyanidin-3-glucoside, while small amounts of delphindin-3-glucoside and pelargonidin-3-glucoside were also detected in some varieties. The content of cyanidin-3-glucoside was closely related not only with the seed coat color, but also with the radicalscavenging activity. The addition of iron reduced by hydrogen had a good effect on the seed coat color to retain the radical-scavenging activity. On the other hand, Fe2O3 had rather negative effects on both the seed coat color and radical-scavenging activity.