Premenstrual syndrome (PMS) is a common disorder. Due to the knowledge lack of the precise etiology of this syndrome, different treatment methods are recommended, one of them is the use of medicinal herbs. This study aimed to investigate the effect of Valerian (纈草 xie c ǎo) root extract on the intensity of PMS symptoms.In this double-blind clinical trial, 100 female students of Islamic Azad University, Tonekabon Branch,Mazandaran Province, Iran, with PMS were randomly divided into groups receiving Valerian (scientific name: Valeriana officinalis) and placebo in 2013. The participants received 2 pills daily in the last seven days of their menstrual cycle for 3 cycles and recorded their symptoms.The data collection tools included demographic information questionnaire, daily symptom severity questionnaire, and a provisional diagnosis of premenstrual syndrome questionnaire.Data were compared previous, one, two, and three cycles after student's intervention using and analyzed by independent t-test, paired t-test, chi-squared test, and repeated measures ANOVA in SPSS 16.A significant difference was seen in mean emotional, behavioral and physical premenstrual symptom severity in the intervention group before and after the intervention (P < 0.001). However, this difference was not statistically significant in the control group. The results of this study showed that Valerian root extract may reduce emotional, physical, and behavioral symptoms of premenstrual syndrome.