Objective:To explore the mechanism of Xiaoying Mixture on autoimmune thyroiditis in view of factor associated suicide / factor associated suicide ligand(Fas/FasL). MethodsFor 58 rats,10 rats were divided into the normal group,and rat model of experimental autoimmune thyroiditis was established in the other 48 rats. Then the successful modeling rats were divided into the model group,tripterygium glycosides group as positive control and Xiaoying Mixture groups with low- and high-does. The level of serum thyroid globulin antibody (TgAb) was detected by radioimmunoassay (RIA); and the expression of Fas and FasL in thyroid epithelial cells were detected by immunohistochemistry.  Results:Compared with the normal group,the level of TgAb in the model group was increased(P<0.05);compared with the model group,the level of TgAb in the tripterygium glycosides group and Xiaoying Mixture group with high-dose was significant decreased(P<0.05). The difference on the positive area of Fas and FasL expression between the normal group and model group was statistically significant (P<0.01); compared with the model group, the expression of Fas and FasL in the treatment groups was decreased with statistically significant difference (P<0.05, P<0.01). For the effect on reducing the expression of Fas,the difference between the tripterygium glycosides group and Xiaoying Mixture groups with low- and high-dose was statistically significant (P<0.01). For the effect on reducing the expression of FasL,the difference between the tripterygium glycosides group and Xiaoying Mixture group with high-dose was not statistically significant (P>0.05); the difference between the tripterygium glycosides group and Xiaoying mixture group with low-dose was statistically significant (P<0.01).  Conclusion:The mechanism of Xiaoying Mixture on cell apoptosis in the treatment of autoimmune thyroiditis may be related to reducing the expressions of Fas and FasL in thyroid epithelial cells