Many young women suffer from premenstrual syndrome (PMS). The purpose of this study was to examine the effect of isoflavone on serum sex hormone and PMS during the luteal phase of the menstrual cycle. We investigated the incidence of PMS in 116 Korean female college students. The incidence of PMS was $19.8\%$. Among them, 9 PMS and 13 control were given 90 mg isoflavone per day during 2 menstrual cycles. The changes in anthropometric measurements, dietary intakes, serum parameters including sex hormones, and PMS symptoms were observed. Isoflavone supplementation did not affect anthropometric measurements. However, SBP and DBP were significantly decreased in control. Dietary intakes were not changed after supplementation of isoflavones. Serum SHBG was lower in PMS group than in control and significantly decreased in control with isoflavone supplementation. PMS symptoms such as backache, sweet, salty, and fatty food craving, coffee/tea drinking, and anxiety were significantly lessened. Based on these results, isoflavone supplementation may benefit young women by reducing some of PMS. Further studies of soy isoflavones effect on sex hormone and PMS may help to prepare for PMS management.