The premenstrual syndrome is a series of physical, mental, and behavioral symptoms with various severities. Different treatments have been introduced for the syndrome due to its unknown complicated causes. Vitamin B1 (thiamin) may reduce mental symptoms of the syndrome through affecting the performance of coenzymes in the metabolism of carbohydrates and main branch of amino acid that plays an important role in appearance of mental symptoms of the PMS.In this double-blind placebo-controlled clinical trial, 80 students with PMS residing at dormitories of Jahrom University were divided randomly into two groups, vitamin B1 and placebo. The severity of the mental symptoms of PMS in two cycles, before the intervention and during the intervention, was recorded by the students. The data were collected using an information collection form, PMS provisional diagnosis form, daily status record form, Beck Depression Inventory. The data were analyzed using descriptive and inferential statistics.The comparison of vitamin B1 group before the intervention with that after the intervention showed that vitamin B1 reduced mean mental symptoms (35.08%) significantly (P<0.0001). Moreover, there was a significant difference between vitamin B1 and placebo groups in terms of mean mental symptoms, as mean symptoms in vitamin B1 group was significantly lower than that in the placebo group (P<0.0001).It seems that vitamin B1 is effective in recovery of mental symptoms of PMS. Therefore, this vitamin can be used to reach a major goal of midwifery, that is, reduction of mental symptom severity of PMS, without any side effects.