Background: Exercise is commonly reported as a factor for decreasing dysmenorrhea symptoms. The aim of the present study was to investigate the effect of six weeks flexibility training on symptoms of dysmenorrhea. Methods: Thirty-four students of The University of Isfahan suffering from primary dysmenorrhea (Age: 20.47 ± 1.54 years, Weight: 54 ± 8.22 Kg, Height: 161 ± 5.34 cm, Body Mass Index: 20.72 ± 2.76) were divided in two exercise (n = 20) and control (n = 14) groups. Exercise group engaged in a 6 weeks flexibility training program include abdomen, back, hamstring and hip adductors stretches. They performed 10 movements, each 3 times, lasting 10 seconds, 4 days per week. One second was added to those movement times each week. Prior and after 6 weeks of flexibility training, all subjects completed menstrual distress questionnaire. Severity of pain evaluate with Visual Analogue Scale. The descriptive and inferential statistic tests like Man-Withney U, Wilcoxon, and independent and paired sample t-test were used for analyzing the statistical information. Findings: After 6 weeks flexibility training, experimental group indicated significant increase in flexibility measurements (back, hamstring, hip adductors) and significant decreases in their mood symptoms (from 14.20 ± 7.42 to 5.35 ± 3.00), physical symptoms (from 15.90 ± 5.93 to 8.75 ± 6.09) and severity of pain (from 6.37 ± 2.69 to 3.79 ± 2.39) (P < 0.001). There were no significant changes in flexibility measurements and dysmenorrhea symptoms in control group. Conclusion: Our findings indicated that selected flexibility training in 6 weeks would decrease dysmenorrhea symptoms. Keywords: Dysmenorrhea, Flexibility training, Pain.