Background: Primary dysmenorrhea is the most common complaint in adolescents and adult young women that disturbs their daily life performance. Aim: The current study investigated the effect of lavender aromatherapy on pain severity in primary dysmenorrhea. Subjects and Methods: This tripleâ€‘blind randomized clinical trial was conducted on 200 students of Ardabil University of Medical Sciences, Iran. Subjects were allocated randomly into intervention (lavender) and control (placebo) groups. The researcher assistant asked the participants to smell the lavender in the first 3 days of menstruation, 30 min in a day in two menstrual cycles. Control group was also administered placebo (diluted milk) to be used as lavender in treatment group. Pain severity was scored by visual analog scale in the first 3 days of menstruation before intervention and 2 months after intervention. Data were analyzed through descriptive statistics and independent and paired samples tâ€‘tests. Results: There was a significant difference in average pain severity between treatment and control groups after intervention. However, students in the treatment group reported significantly less pain severity 2 months after intervention (P < 0.01). Conclusion: Using lavender aromatherapy for 2 months may be effective in decreasing the pain severity of primary dysmenorrhea.