Abstract： Objective： To observe the clinical effect of Yimucao granules for primary dysmenorrhea. Methods：A total of 100 cases of patients with primary dysmenorrhea were selected and divided into two groups according to the random number table method. The control group which has 50 cases was treated with ibuprofen sustained- release capsules， and the study group which has 50 cases was treated with Yimucao granules. Both groups were treated for three menstrual cycles and followed up for three months. Clinical effects， incidence of adverse reactions and recurrence rates were compared between the two groups. Before and after treatment， symptom scores and levels of platelet activating factors(PAF)， prostaglandin F2α (PGF2α) and β- endorphin were compared between the two groups. Results：The total effective rate was 96.00% in the study group， higher than that of 82.00% in the control group(P＜0.05). One month after treatment， symptom scores in the two groups were decreased when compared with those one month before treatment(P＜0.05)，and the symptom scores in the study group was lower than that in the control group(P＜0.05). After treatment for one and three menstrual cycles，levels of PAF and PGF2α in the two groups were decreased when compared with those before treatment(P＜0.05)，and β- endorphin levels were increased(P＜0.05)； levels of PAF and PGF2α in the study group were lower than those in the control group in the same period(P＜0.05)， and β- endorphin level was higher(P＜0.05). The incidence of adverse reactions was 8.00% in the study group and 6.00% in the control group，there being no significance in the difference(P＞0.05). During three month follow-up，the recurrence rate was 2.08% in the study group，lower than that of 14.63% in the control group(P＜0.05). Conclusion：Yimucao granules for primary dysmenorrhea is safe and effective，with a low recurrence rate. Its mechanism maybe related to the down-regulation of levels of PAF and PGF2α and the promotions of β-endorphin secretion.