Objective To observe the effect and possible mechanism of Zuogui Wan on fetal rats birth weight of gestational diabetes mellitus（ GDM） rats. Methods Pregnant SD rats were randomly divided into normal control group

GDM group

insulin intervention group

Zuogui Wan intervention group and combined intervention group

10 rats in each group. On the day of conception

rats of these groups were injected intraperitoneally with streptozocin 45 mg / kg to establish GDM rat models except rats of normal control group. From the third day of conception

Zuogui Wan intervention group and combined intervention group were given Zuogui Wan 10 g /（ kg·d） by gavage. Normal control group and GDM group were given equivalent distilled water by gavage

once each day. Insulin intervention group and combined intervention group were given Novolin R 20 IU /（ kg·d） by subcutaneous injection

once each day

until 18 days of conception. On the 19 th day of conception

rats were anatomized. Weigh fetal rats

detect serum corticosterone

placenta 11β-hydroxysteroid dehydrogenase 2（ 11β-HSD2） mRNA and the expression of protein.Results Compared with normal control group

serum corticosterone in GDM group increased; fetal rats weight

placenta11β-HSD2 mRNA and the expression of protein decreased（ P < 0. 05 or P < 0. 01）. Compared with GDM group

serum corticosterone in insulin intervention group and combined intervention group decreased; fetal rats weight increased（ P < 0. 05 or P < 0. 01）. Fetal rats weight in Zuogui Wan intervention group had no significant difference（ P > 0. 05）. Placenta11β-HSD2 mRNA and the expression of protein in combined intervention group increased（ P <0. 05）. Conclusion Zuogui Wan could improve the effect of GDM pathological state on fetal rats weight. It was more effective to combine with insulin. The mechanism may relate to up-regulate the expression of 11β-HSD2 and decrease the concentration of corticosterone.