Introduction & Objective: Premenstrual syndrome (PMS) is a common  cause of substantial psychological and physical distress for women during their reproductive years that occurs luteal phase  and  reduced their quality of life . Despite the magnitude of this problem, there was no effective treatment for PMS. Evening primrose oil was used traditionally for PMS but the cure effects are still ambigous. The purpose of this study is to determine the effect of Evening primrose oil on premenstrual syndrome.

Materials & Methods: Randomized ,double blind .placebo controlled, parallel group comparison over two menstrual cycle on 76 women suffering premenstrual syndrome, aging 18-40 , going to 10 medical centers in Isfahan. volunteers underwent a preliminary screening interview, completed Daily Symptom Rating ,for two cycle ,and attended a medical screening visit before being diagnosed with premenstrual syndrome and screening for depression by Beck questionary.76 women evaluated (active :38 , placebo:38).participant took evening primrose oil ,daily 3 capsule and matching placebo for two cycle .The data collecting tool was a Beck , Daily Symptom Rating and demographic questionnaire. Data were  analysed by SPSS and descriptive and analytic statistics (chi-square, t test , mann-whiteny u, fisher exact test , and willcoxon)

Results: The finding showed there was a significant difference between mean symptom severity in Evening primrose oil therapy group before and after the treatment (p<0/01). But there was no significant difference between mean symptom severity in control group before and after the treatment (p<0.23). Also there was a significant difference between mean symptom severity in Evening primrose oil  therapy group and the control group (p<0.02).

Conclusions: Based on the finding of this study, Evening primrose oil has been effective on the improvement of premenstrual syndrome. In this case, more study repetition long perseverance is needed for achievement of more strong witness.