Objective To study the effects of soybean isoflavone (SI) on lipid peroxidation and liver ultrastructure in ovariectomized rats. Methods Seventy female SpragueDawley(SD) rats were randomly divided into 7 groups according to the levels of total cholesterol(TC) in serum: hyperlipoid group, estrogen group, lowdose SI group, middledose SI group, highdose SI group, sham group and normal control group. After bilateral ovaries were extirpated except sham and normal control groups for a week, the estrogen, different doses of SI or deionized water were fed with intragastric administration for 12 weeks. The rat body weight was weighted once per week and blood samples were collected at the time of ovariectomization, the 4th and 8th weeks after administeration at the time of being killed. The serum TC, triglyceride(TG), high or low density lipoprotein-Cholesterol(HDL-C, LDL-C), lipoprotein(a) and antioxygen enzyme were assayed. Results LDL-C levels in SI intervention groups were significantly lower than that in hyperlipoid groups but higher than that in normal control group; the levels of lipoprotein(a) were changed; there was almost no effect on HDL-C in serum. SI could retain integrity and ultrastructure of hepatocyte; there was similar structure in high dose SI group and normal control group. An obvious damage was detected in hyperlipoid group. Conclusion SI can improve lipoprotein(a) and lipid peroxidation in ovariectomized rats. A continuous intervention w