Objective This study is aimed to investigate the effect of soy isoflavones on initial oestrus and some physiological characteristics of female mice． Methods Total two hundred and ten 3-week old female ICR mice were randomly divided into 3 groups，and fed with the control (0 mg / kg) ，low soy isoflavone level (50 mg / kg) and high soy isoflavone level ( 400 mg / kg ) diet separately． The state of vagina opening and vagina plug after treatment with soy isofalvones，the concentration of estrogen in their serum on Day45 and Day65 as well as their body，uterus and ovarian weight， were analyzed and recorded separately． The ER ( estrogen receptor ) on uterus was detected by immunohistochemical staining． Results There were significant differences of the durations for murine vagina opening between those treatment groups and control one． The body weight of two treatment groups was significantly lower than that of the control one． The positive rate of ER on the stroma in control group was significantly lower than that of the other treatment ones，but that on the glandular epithelium in high level group was much higher than the other groups’，and there were no significant differences on the luminal epithelium among all the groups． Conclusions Collectively，it can be concluded that the 400mg / kg soy isoflavones in the diet can simulate murine vagina opening，increase their body weight and degrade the estradiol level in their serum on Day65． affect the expression of estrogen receptor on their utrine stroma， luminal epithetlium after 8 weeks． however，50mg / kg soy isoflavones in the diet only affect the body weight of 45d，and the expression of estrogen receptor on their utrine stroma after 8 weeks