Medical therapies have been widely used for premenstrual syndrome (PMS), but in all of them side effects are predominant. Herbal remedies rarely have side effects and people have more tendencies toward them than chemical therapies. In this study the therapeutic effect of Vitex agnus castus on women who had the PMS, in comparison with placebo, were investigated. In this randomized, placebo-controlled, double-blind study, from 134 selected patients 128 women suffered from PMS were evaluated (active 62, placebo 66). All patients answered to a self assessment questionnaire about their headache, anger, irritability, depression, breast fullness and bloating and tympani during the premenstrual period before the study. Forty drops of Vitex agnus extract or matching placebo, administrated for 6 days before menses for 6 consecutive cycles. Patients answered the self-assessment questionnaires after 6 menstrual cycles, again. Each item rated using a visual analogue scale (VAS). The mean age was 30.77 (SD=4.37) years in the active group and 30.89 (SD=4.02) years in the placebo group.Rank of variables had significantly difference in active and placebo group before and after the study (P<0.0001) also we noticed significant differences on the use of Vitex agnus in comparison with placebo (P<0.0001). Vitex agnus can be considered as an effective and well tolerated treatment for the relief of symptoms of mild and moderate PMS.