Abstract: In order to investigate the effect of dietary active selenium supplementation on reproductive hormones of ewes in estrus cycle, 20 Mongolian ewes at 2 ～ 3 yr old, with good body condition, randomly divided into two groups. The control group fed with basic diet, while the experimental group fed with diet containing 1% active selenium. The trial period was 36 d with the pre-trial period of 15 d and the formal trial period of 21 d. After 5 d of pre-feeding, every ewe was given intramuscular injection of 1 mLof chloroprostanol（PG）, and then injected with 1 mLof PG at the day 13 of trial. Every ewe was given vaginal suppository at the day 19 of trial; The vaginal suppository was withdrawn and 400 IU of PMSG was injected intramuscularly at the day 30 of the trial. Six ewes in each group were randomly selected to collect the serum samples at the day before the day of embolectomy, 1 st d, 10 th d and 20 th d of estrus cycle, and the contents of serum E₂,P₄,LH and FSH were determined by radioimmunoassay. The results showed that compared with the control group, the contents of E₂,P,LH and FSH in serum of ewes in experimental group were increased.During the oestrus cycle, the content of serum E₂ was found the highest at the beginning of oestrus（P＜0.05）, the content of serum progesterone was at high level at the day 10 of oestrus cycle, the content of serum LH and FSH was at high level in the beginning stage of cycle, and then decreased. It could be concluded that dietary active selenium supplementation would promote the secretion of reproductive hormones in ewes.