Abstract Background: Primary dysmenorrhea is the most prevalent menstrual problem in young females, which affects their daily activities and quality of life. Although some studies have demonstrated an association between BMI and the incidence and severity of dysmenorrhea, this relationship is still contro-versial. Aim of Study: To investigate the effect of body mass index on primary dysmenorrhea and daily activities in adolescents. Subjects and Methods: A correlational study involved 100 adolescent girls, selected from Beni-Mazar Secondary School throughout the period from March 2019 to August 2019. Their age ranged from 16 to 25 years. Subjects were classified into 4 groups equally according to body mass index. Pain intensity was measured using numeric pain scale, pressure pain threshold using pressure algometer and daily activities using daily activities questionnaire. Results: The study findings revealed that there was a significant difference in the numeric pain scale between groups (p=0.001), there was a significant difference in pressure pain thresholds between groups (p=0.0001) and there was no significant difference in the daily activities between groups (p=0.17) except for school absenteeism and social activity there was a significant difference (p=0.001), (p=0.006) re-spectively. Conclusion: The difference in body mass index had an effect on primary dysmenorrhea; both obese and underweight subjects suffer from primary dysmenorrhea more than subjects with normal weight and overweight, there was no difference on daily activities except for school absenteeism and social activity had higher percentage in obese and underweight.