1. Small seeded varieties of Spanish and Valencia types showed only shallow dormancy just after harvest, but large seeded varieties of Virginia type showed deep dormancy continuing during rather long period.2. Differences of nature of dormancy due to difference of growing areas between Japan and Israel could hardly been observed.3. Preheating for two to three days at 60°C or seven to 14 days at 45°C was most effective for breaking dormancy of peanut seeds. This treatment is usable for routine works.4. Entire or partial decoating is much effective for breaking dormancy. Presoaking also induces rather high germination. Accordingly, the cause of dormancy of peanut seeds seems to be due to germination inhibitors contained in the seeds. These inhibitors are thought to be contained in both seed coat and embryo during some period after harvest, but to become soon contained only in seed coat.