Abstract: To investigate the effect of estrogen receptor(ER) on early embryos of generic hybrids of chicken-quail.we acquired cross bred eggs of chicken(♂)-quail(♀) by artificial insemination,hatched the eggs in one batch according to the standard condition of chicken,collected early living-embryos of 2.75,3.00,3.25,3.50,3.75,4.00,4.25,4.50,4.75 and 5.00 d at random.Adopting RT- PCR and multi-ply PCR， we identified the sex of embryos with primers of Wpkci and β-actin,then selected four female and male embryos respectively from each period taken β-actin as internal standard to determined the relative quantity of ER mRNA.The patterns of ER mRNA expression of male and female were basically consistent.The expressive level were all higher in 2.75～3.00 d,declined distinctly in male(P<0.05) and very significantly in female(P<0.01) in 3.25 d,then ,all reached initial level in 3.50 d and declined great significantly in 3.75 d (P<0.01),then maintained the lower level.Compared the expression between different sex of the same periods, ER mRNA expression of female were higher than male in 2.75～3.50 d ,and the difference was very significant (P<0.01). We found 2.75～3.50 d were the gonadal differentiation period.The abnormal ER expression may induced embryos death and hybrid infertility in the periods of gonadal differentiation.Those conclusion should be confirmed after further research on ER expression and function in chicken and quail’s embryos respectively of the same period.