Objective To investigate the effect of the Shaofu zhuyu decoction on the changes of the body writhing behavior,inflammatory reaction and COX-2 expression of the rats with the primary dysmenorrhea. Methods Fifty SD female rats were randomly divided into blank control group,model control group,high,middle and low dose of Shaofu zhuyu decoction groups,10 in each group.Dysmenorrhea rat model was established by treating with estradiol benzoate and oxytocin.Effect of Shaofu zhuyu decoction of different doses on writhing behavior,changes of endometritis cells and COX-2 expression in uterine smooth muscle of dysmenorrhea rats were observed. Results In the model control group,latency of the body writhing behavior was shortened and the total score was high,many inflammatory cells (especially for neutrophils) infiltrated in endometrium and uterine smooth muscle,immunohistochemistry showed that brown granules were found in the cytoplasm of smooth muscle cells of uterus,and COX-2 expression was positive in uterine smooth muscle cells.As compared with the model control group,writhing latency increased,total score decreased in both the high and middle dose of Shaofu zhuyu decoction groups,and infiltration of a small number of inflammatory cells was seen in the endometrium and smooth muscle(P<0.01);COX-2 expression was decreased (P<0.01) especially for the high dose of Shaofu zhuyu decoction group.As compared with the model control group,the latency of the low dose of Shaofu zhuyu decoction group was significantly increased,and the total score was decreased(P<0.01),but there was no significant difference in inflammatory cell infiltration and COX-2 expression (P>0.05). Conclusion A potential mechanism by which Shaofu zhuyu decotion treats primary dysmenorrhea may be related with alleviating pain,inhibiting inflammatory responses,and down-regulating expression of COX-2.