Abstract: Twelve Simmental heifers (average BW 380 kg, aged 1.5) consuming a corn straw diet were chosen and allocated into four treatments and supplemented with four levels (0, 0.3, 0.6 and 0.9 mg Se/kg of DM) of supplemental sodium selenite to evaluate the effects of selenium on reproductive hormone excretion during the oestrous cycle. The results showed that serum estradiol, luteinizing-hormone, follicle-stimulating hormone and progesterone concentrations were increased significantly (P <0.05) by supplemental sodium selenite at 0.3 mg Se/kg of DM than control group. Serum estradiol, estriol, luteinizing-hormone, follicle-stimulating hormone and progesterone concentrations were increased by supplemental selenium of different selenium sources at 0.9 mg Se/kg of DM, but no significant difference with control (P >0.05). This indicated that selenium sources had positive effects on reproductive hormone during estrus cycle by supplemental selenium at 0.3 mg Se/kg of DM.