Abstract: Objective To observe the effectiveness of RU486 in the treatment of OHSS model rat. Methods A total of 30 immature female rats aged 22 days were injected s.c. daily with 10 IU pregnant mare serum gonadotropin （PMSG） for 4 consecutive days from the 22 nd to 25 th day of life. On the 26 th day of life,100 IU human chorionic gonadotropin （HCG） were injected s.c. to produce OHSS model. On the 27 th day of life, 30 rats were randomly divided into five groups. Each group contained 6 rats. The rats in group Ⅰ as control group were not given Ru486 treatment； in group Ⅱ to group Ⅴ were given s.c. with the dosage of RU486 1 mg/kg, 5 mg/kg, 10 mg/kg,20 mg/kg respectively. Fourty-eight hours after RU486 treatment, the ovarian weight, peritoneal capillary permeability and ascites score of all rats were determined. Results The ovarian weight in group Ⅲ and Ⅳ was 0.321±0.027 g, 0.302±0.035 g, respectively, which were significantly lower than that of control group （0.352±0.022 g）（P＜0.05，P＜0.05 respectively）, peritoneal EB content in group Ⅲ and group Ⅳ was 1.18土0.66 μg and 1.17±0.47 μg, respectively, which were significantly lower than that of control group （2.33土1.06 μg） （P＜0.001,P＜0.001 respectively）,and the ascites score in group Ⅲ and group Ⅳ was 2.08±0.38, 1.67±0.48，which were significantly lower than that of control group （2.67±0.41） （P＜0.05,P＜0.01 respectively）.However, there was no significant difference to be found between group Ⅱ and Ⅴ and control group. Conclusion Optimal dosage of RU486 treatment could improve the condition of OHSS in the rat model.