Background & Aim: DYSMENORRHEA is a common problem in young women. Although DYSMENORRHEA is not life threatening, it can have adverse effects on quality of life. Various treatments are offered for DYSMENORRHEA including vitamin E supplements. This study investigated the effect of vitamin E on PRIMARY DYSMENORRHEA.Methods & Materials: In this before-after clinical trial, 200 female students who suffered from PRIMARY DYSMENORRHEA were recruited to the study at Mashhad University of Medical Sciences. First, they filled a questionnaire including demographic characteristics and the pain measurement chart (duration and severity). Then, they were divided into three groups: low, moderate and severe DYSMENORRHEA groups. Vitamin E (200 mg) tablets were administered to them and they were requested to have them once/day for five days (two days before and three days after the beginning of the menstruation). They filled the pain measurement chart (duration and severity) after two treatment periods. Data were analyzed using t-student, chi-square, and variance analysis tests in SPSS software.Results: The age of the students ranged 19-26 years old. The severity of DYSMENORRHEA was mild, moderate, and severe respectively in 28%, 60%, and 14% of the students. The mean of painful menstrual days was 2.05 in the first month, and 2.08 in the second month before using vitamin E. After using vitamin E, painful menstrual days were 1.63 in the first month and 1.64 in the second month. Regarding duration of painful days, all of the participants felt pain in the first day of menstruation. After using vitamin E, about one sixth of the participants in the first month and one fifth of them in the second month had no pain at first day. The severity of pain was 5.18 before using vitamin E that decreased to 3.40 after the intervention.Conclusion: Vitamin E administration in women, who suffer from PRIMARY DYSMENORRHEA, significantly reduces the severity and duration of pain. It can be used as an alternative treatment method in affected women.