Isoflavones are associated with prevention of chronic diseases such as cancer, osteoporosis, and hypercholesterolemia. The isoflavone contents at different development stages of soybean were anayzed to examine the period of their formation and reaching maximum level. Soybean sedds have been collected from 13th to 21st week after sowing. Total daidzein and genistein contents increased from 115 and 37mg/kg at 13th week, to 795 and 699mg/kg at 21st week, respectively. Total glycitein content showed less remarkable change during seed development than other isoflavones. Thus mature soybean may have maximum preventive effect from chronic diseases since it contains the highest level of isoflavones.