To estimate the preventive potential of Japanese traditional cereals against oxygen radical-related chronic diseases such as cardiovascular diseases and diabetes, antioxidant and radical-scavenging activities in the extracts of five Japanese traditional cereal grains were analyzed by using an assay system of lipid peroxidation and a radical compound, 1,1-diphenyl-2-picrylhydrazyl (DPPH). DPPH radical-scavenging activities in the extracts of Japanese cereal grains were divided into two groups. One group including Japanese sorghum, black rice and red rice showed strong radical-scavenging activities, but the other group including Japanese barnyard millet and foxtail millet did not exhibit significant radical-scavenging activities. The DPPH radical-scavenging activities of these extracts were closely correlated to the contents of phenolic compound in the extracts, but not to the sugar or protein content in the extracts. In contrast, all the methanol and water extracts of the cereal grains caused significant antioxidant activities against hydroperoxide generation in the peroxidation of linoleic acid, in which the water extracts of these cereal grains caused much higher antioxidant activities than the methanol extracts of the same cereals. These results suggest that Japanese traditional cereals contain qualitatively different principles associated with antioxidant and radical-scavenging activities, and possible principles responsible for the antioxidant and radical-scavenging activities in the cereal grains are discussed.